With the new alpha 3.9 its not as viable a strategy to assault london on G3 (this was the point of alpha 3.9)
My guess is sealion on G4 or no sealion at all
If you dont do sealion you still need to wipe out UKs fleet (in atlantic) by end of G2 and have subs in convoy zones to maximize IPC loss for UK.
Honestly if UK goes for a (atlantic) fleet build early then he is leaving London open to sealion on G3 or G4