It wasn't, really. The retention of the skilled/trained personnel of the BEF meant that the British had a core around which they could built their force for the return to Europe, but it had very little influence on Sea Lion itself, which was a stupid idea (such as it was, the Navy and Army never did agree on a plan) and would never have worked no matter if the BEF survived or not. Bernard over at Military History Visualised goes over the various reasons why Sea Lion was a non-starter. https://www.youtube.com/watch?v=i_7Mn9CxKkk

British Army in July 1940 had, on paper, 600,000 infantrymen in the UK. Granted, almost half were Home Guard and not up to much. I don't know of any stats which show by job description what numbers of BEF got back, doing a quick review of the Orbat, I count the BEF had about 86 infantry battalions. Each infantry battalion had four rifle companies of 97 men, so assuming they were all up to strength, that's 33,000 of that figure who were infantrymen. So, the British basically had a quarter million somewhat professional, if disorganised, infantry to oppose the landing even if the BEF was left behind. Given that there was bugger-all chance of the Kriegsmarine sustaining a landing in the face of the Royal Navy, it was down to the 25,000 or so airmobile troops the Germans could provide. Even the Home Guard could deal with that.