The assertion was made by Stuart Laycock in his pop history book 'alll the countries we've ever invaded' which traces where exactly british army units have fought and found 22 of the modern 197 countries (it was written before the independence of south sudan) where he could find no trace of british troops ever having fought (discounting aerial combat).

The problems with taking that figure out of context is, firstly he's talking about geographical areas not political units. the UK has never invaded the Republic of India or the Republic of guatemala but they have fought in those areas when they were controlled by previous governments. If you're talking about the modern political units, the number is much much lower.

Secondly, he includes incidents such as the d-day invasion of France where the Uk was invading a country to free it or assist it against foreign troops. It's about where british troops fought not why they were there. (Albania for instance is a country where the UK has fought, in both world wars, but has never tried to annex)

Thirdly, he includes purely naval encounters which saw no land troops deployed (such as algeria, which the british navy spent a lot of time bombarding to convince the barbary corsairs to release british slaves but where land troops weren't deployed until ww2), even when these encounters were done by privateers rather than official naval units which some people think is a stretch (Ecuador for instance was attacked repeatedly by british pirates carrying a letter of marque allowing them to attack spanish targets but not by the actual navy).

Fourthly, he fully admits in that book that he doesn't know for sure on a lot of stuff. So for every entry of the 22 where he couldn't find evidence of british troops operating, he goes well maybe they did (so for luxembourg, it was american troops who invaded them in ww2 but possibly some british troops accidentally crossed the border and at least one airman was shot and killed over it and for belarus, english knights invaded lithuania in the 14th century and maybe they fought in modern belarus).

So if you're talking about which of the modern day countries we know for sure had british troops or naval units operating in their claimed land at some point, then yes it's accurate. If you're taking it out of that context and trying to make a larger statement about it then no, not really.

Lots of the incidents Laycock talks about are not really invasions. The Burkina Faso entry is about an army sent to assist a native kingdom against the french which turned around before fighting a battle once a peace treaty was signed with the french. The Brazil entry is about a naval unit sent to blockade a brazilian port as part of a diplomatic crisis that was resolved bloodlessly. For Hungary it's British units fighting in the Austrian Army that liberated it from Turkish control, etc.

It's accurate in the question he is asking, but the question he is asking might be not what you think it is if you see that answer out of context.