There are three J lounges at Heathrow. The best is at T5B, you can go there regardless of which gate your flight departs from, if you needed to go back to T5A you MUST use the tunnels not the train, or you'll be forced landside and end up in all sorts of trouble. Yes, fizz is available, I think champagne on demand, ask a server.

Club kitchen isn't all that great - you can use it when seatbelt signs are off but don't expect too much. It'll be in one of the galleys. Crisps, the odd chocolate bar, not a great deal.

Business Class is a perfectly nice way to travel, enjoy yourself, but don't set your expectations to unalloyed luxury level!