The fighters need to get out of uk or he needs to transport units off for you to be able to do sea lion (assuming he buys 9 inf or 6 inf 1 fighter to defend london). If he buys elsewhere on the map sure by all means go for it.
Make sure your Italy can bomb the airbase on round 2… in case he tries to repair it after Germany’s turn.
Make sure you get 1 destroyer or he can stop your attack with subs.