VS Arrival Lounge in LHR
 

Don't know if the question has been asked before, but I will be flying J from HKG to LHR connecting onwards to BA from LHR to BCN. My understanding is that BA flys out from the same terminal as VS. Wonder if it's possible to visit the revival lounge for the transit as I will have no lounge access with BA flying Y.

You can - but the lounge is landside, so you will need to pass through immigration/customs etc, visit the lounge, then go back to departures for your onward flight. This is doable depending on how long your connection is.

Connection is 3 hours, but i believe if in transit, i do need to collect my bags and go through immigration anyways given BA and VS don't have interline agreement? Unless I'm mistaken?

VS do have an interline agreement with BA, so they will check bags through if you are on the same ticket. But on two separate tickets, that won't happen and you will need to collect them and re-check.

The interline agreement between VS and BA (and VS and most other interline partners) can be a bit of a challenge to use at times. Many agents - especially at outstations like HKG or IAD, are relucatant to add onward BA flights to your checked bags. Others dont know how to do it. Others will tell you it's simply not possible. This is the type of issue you should be prepared for pushback on, and be ready to stand your ground on - escalate to a supervisor if need be. It is 100% doable, but agents at outstations aren't always the most well-versed in the implementation of the interline agreement.

Interlining bags on single tickets is a day to day activity and should not cause problems. Even on a BA-VS itinerary (supposed the arch enemies!). And especially not at major airports such as HKG or IAD.

Where people are travelling on separate tickets then many airlines refuse to interline. Indeed for example BA won't interline even when the next flight is its their own metal. Ditto AA/ So you won't find them interlining on separate tickets to anyone else either.

In those cases standing your ground or pushing back simply won't help you. The airlines have instructed their staff not to do it.

It is not clear what the position of the OP is. They use words like 'connecting' and 'transit' which are meaningless without the correct information on the ticketing arrangements they hold with VS and BA.

Re Revivals on separate tickets they could use it after collecting their bags and either check them in straight away with BA and then go to Revivals or take them with them, use revivals and then check them in bearing in mind the BA bag drop deadline. On a single ticket with bags interlined they would clear the UK border then go to revivals. They would obviously have more time in that case.