The 2 main ones are 'Ancestry' and 'Find my Past'.
They both have advantages & disadvantages, and both have access to different records.
Ancestry is more 'intuitive' to build up your family tree etc and will give hints of records or other people's trees to look at.
It has a large record database including things like WW1 service records (80% of them got destroyed in the WW2 blitz)
The main problem with it now though is too many used it thinking all their family records would be on it, and adding people to their trees just because they were a 'best fit', based on the info they had, rather than getting confirmation from other sources. People therefore made mistakes and other people copied the info without checking and assuming it was correct, compounding the errors.
Find my Past is similiar although i believe it hasn't as large a records database as Ancestry.
It does WW2 casualty lists, but you need to take out a 12 month subscription to access them, and i think the 1939 register which Ancestry doesn't have.