5 British tanks in your garage at one time is the requirement.
Tank type doesn't matter, however with your starting collection of 7 tanks, and only 3 more slots, you'll want to do some planning on what to sell.
I recommend getting all your starting tanks to Elite status by researching all their stuff, then selling one of them you disliked the most. You should now have enough room for your 5 British tanks! Just buy all the Tier 2 tanks and you're set!
There is a daily limit of 5 tanks sales however, so be careful you don't sell and buy the wrong tanks or you'll be stuck having to wait a day or buy extra tank parking spots with gold.