Renting and Building Hours
ChiSau The Poet is correct that getting into a syndicate is the cheapest way if you are going to fly every couple of weeks or more. That's something I ought to do but meanwhile fly with a Club.
Another thing from your point of view is where you want to fly. I trained at Luton (amongst others) and so know that local area fairly well. Now I fly at Fairoaks where we are all PA-28 (which is probably fairly equidistant from central London to Denham/Elstree for you) and have a completely different area to fly in .... south and west of London and along the South Coast. That gives me somewhere different to fly and new scenery to explore without long familiar transit flights.
Type checks with us aren't - in my experience - a big deal, the fourth next most important thing .. after safety, safety and safety of course .. is familiarity with the area so you don't infringe LHR and GAT.
If you wanted to use 2 clubs how about Denham and Fairoaks 2 get 2 different starting points ????
I have got my time pretty much split between Cessnas and Pipers and we have use of a 172. Although I haven't flown one since GumBOIL at Barton about 3 years ago, I don't anticipate any problems doing a quick type check and check ride to get "authority" to fly it.