Let's assume I am an able bodied man or a woman in their twenties with an university education and speak fluent english. Maybe I left Germany because I am a communist or a social democrat. Could I join the army as an officer or translator? Could I work for intelligence services? What would my experience be?
Let's say you went to Britain, and found yourself employed at a university or some other professional, respected position. Let's say you have a chair at Oxford, in languages or engineering or some obviously useful field, are ethnically Jewish, and had been a member of the Social Democratic Party. A pretty clear enemy of Hitler, with skills that would be great to have, right? It wouldn't make any difference in those early months of the war.
As the Wehrmacht was invading Poland in September 1939, the British War Office hurriedly established a number of ‘enemy alien tribunals’ in order to judge the loyalty of the resident German and Austrian population. Initially the tribunals had neither a uniform system of judging this loyalty nor a sensible approach to managing the numbers of ‘enemy aliens’ under review, but the system was gradually improved upon and by February of 1940 the vast majority of ‘enemy aliens’ had been brought before the tribunals. You, as a recent immigrant/refugee from German, would be among them.
Of the seventy-five thousand German and Austrian nationals living in Britain, over 88% were revealed to vehemently oppose the Third Reich and held no desire to see Britain defeated by their countrymen. Less than six hundred aliens were considered a potential threat to the war effort, henceforth designated ‘Class A’, and immediately arrested and interned. The roughly sixty-six thousand opponents of Nazism were labelled as Class C, considered harmless, while the remaining seven thousand were considered Class B, those allowed their liberty with some restrictions on employment and travel. Many of these tens of thousands would have been like you: Germans who had fled National Socialism, refugees by necessity of their race or their political or religious beliefs.
Three months later, the situation on the continent dramatically changed, as the Netherlands, Belgium, and France were quickly overrun. A barrage of false but terrifying reports from those evacuated from the continent were published in the national press. These reports, such as that of the former British envoy to the Netherlands, Sir Nevile Bland, made bold and detailed claims about the extensive use by the German army of parachutists aided by ‘fifth columnists’ planted in advance. Widely published, Bland’s account gave the judgement that “every German or Austrian” in Britain presented “a real and grave menace,” and declared that “all Germans and Austrians, at least, ought to be interned at once.” Despite the alien tribunals having previously concluded that only a small minority of ‘enemy aliens’ within Britain were potential dangers, Churchill’s government agreed with the supporters of mass internment found in the press and in Whitehall, with Churchill himself giving the order to "collar the lot". By June 1940, the bulk of enemy aliens that only weeks before had been deemed harmless to the war effort had been taken from their homes and dispersed across the country, the vast majority of internees being deposited on the Isle of Man, while a significant number were deported from the British Isles to the Commonwealth.
Let's say you went with the bulk of the enemy aliens to the Isle of Man, along with some British fascists, IRA members, and pacifists. So there you are, sat in a requisitioned boarding house in one of Britain's most eligible holiday destinations, surrounded by armed guards and barbed wire. Once the initial panic died down, and it became clear to the authorities that 1. They had overreacted and 2. There was no evidence of fifth columnists being used by the Germans, people began to be released. It wasn't soon enough for those internees who had been killed in the sinking of the Canada-bound Arandora Star, or beaten, robbed, and worse on the Dunera en route to Australia, but it was something. It would still take months for everyone to be freed, and some stayed until the final months of the war.
Your time behind barbed wasn't all that bad, though. You have a university education, you say? What about lecturing at the Hutchinson Camp University? Or sit in on a talk given by some of the most renowned European scholars, caught up in the panic just like you? The same camp had regular performances from musicians and exhibitions by artists (Hutchinson Camp was known as the Artist's Camp due to how many were held there.) Whichever camp you were in, you had access to the seaside, to sports, to the cinema (all under armed guard, although at times the guards had no ammunition). The range of religious faiths among the internees meant that a preacher of your denomination or language was probably interned, and if they weren't then efforts were made to try and provide someone from outside the camps. If you were married, then maybe your wife would also be interned. If you were incredibly lucky, you might be interned together at Rushen Camp, but otherwise there were visits arranged.
But you say you want to help the war effort, and you have useful skills? Then your chances of release are much higher. You're right to think that translation work would be a priority, and many former internees were employed in this. Intelligence and security work was certainly possible, but much rarer. You could serve in the forces, but most of the time this was limited to non-combat roles like bridge construction and logistics, although far from always, and there are plenty of cases of former internees fighting in the war. Likewise, if you worked in industry you might find yourself forbidden to work in munitions or anything particularly sensitive, and mostly this came down to trust.
Edit: I've focused mainly on the internment, and less on what followed, because internment was the focus of my research. I'm also on mobile, and on an overcrowded, overheated train, so my formatting or phrasing may be a little off.
I should also make clear that while internment on the Isle of Man wasn't the worst fate, and many looked back fondly on their "involuntary holiday", but this was still imprisonment without trial. People were, at a minimum, separated from their families for months at a time.
Edit 2: Sources: