Aside from the tight electoral system that would have contained a strong CPGB, can I introduce some possible factors.
First for this scenario to work you might have:
The CPGB would have to soften some of its policies such as accept parlt democracy (for the while) and be quite nationalistic. Both fairly possible.
You could see the CPGB replacing Labour in the English inner suburbs of the cities, Wales and Scotland as a proto nationalist party and maybe some rogue and disgruntled rural areas where there is plenty of erm rogue and disgruntled farm workers.
Take a rough figure of 5-10% nationally and you can see that it would instantly weaken Labour, strengthen the Tories.
Although you might have seen a revival of the Lib-Lab electoral coalition. Rememeber from the 30s to the late 60s, the Liberals were very much a local and thinly supported party.
This CPGB would have thrived from 1942 to say the Budapest uprising, then decline for a short revival during the 70s and early 80's, before being splittered and absorbed, as has happened for so many other European Communist Parties.
But can i say that the ability of the CPGB to be as electorally strong would not have come from the Communist themselves, as they were a fairly parochial bunch of people who were increased in negative politics (its the class system, its capitalism). Also more devoted to their industrial policies, ie taking control of the unions. A strong CP would ahve emrged from voters seeing them as the fulcrum of their political aspiratios than any thing CP members would or would have be able to do.