As often with these sorts of decisions it comes down to 90% convenience and 10% solid reasoning; Elstree (whether it be Cabair or any one of the other schools there) is your best bet for convenience, and with the weather being what it is a school that's only five minutes from your front door is better than one that's an hour away. I won't tell you how many times I've driven for an hour to get to my home airfield 'on the offchance' and been sorely disappointed when the weather didn't improve by the time I got there. The hour's drive back isn't fun under those circumstances.
On the other hand, Elstree is busy, which means that it'll cost hard cash in terms of having to wait at the hold for ten minutes (but your hour runs from breaks off, remember), you may not be able to book a/c when you want to, it's a hectic bit of airspace to learn in. The advantages? It's a hectic bit of airspace to learn in, you get very good RT experience, you get used to operating from a busy airfield, etc.
Personally, though, I've gone for the slightly more laid back approach. There are a couple of airfields within, I'd say, half an hour's drive of Elstree, and that's Stapleford, just off the M25 on the junction with the M11, and High Cross, a little way up the A10 in Hertfordshire. I fly out of High Cross with a gentleman called Alan Adams. The airfield's unlicensed, so I have to fly ten minutes down to Stapleford to start the clock ticking, and it's a rather small operation with one a/c in service. Alan's a career instructor, and a pleasure to fly with, and the experience of having to fly back and forth to Stapleford every single lesson is valuable, particularly when you start on your navs. You also never, ever, have to wait for a take off slot. You still get to fly through fairly busy airspace - you're still under the London TMA, you have Stansted just to the Northeast and Luton to the Northwest - but it costs an awful lot less... Even Stapleford, which is busier, doesn't seem to have the delays you might get at Elstree, and again you'll still get the experience of operating into a relatively busy airfield under very busy airspace. Perhaps the most important bit, though, about High Cross, is that it is very laid back. Come back from a couple of hours in the air, put the kettle on, sit back, chat for a while before going home. No pretensions, no formality, very relaxing.
All down to personal preference, as always. There are advantages and there are disadvantages - best thing to do is visit a few places before committing yourself. And don't underestimate the value of personalities; you'll be spending an awful lot of time cooped up in a very small space with your instructor, so it's worth getting to know them a bit.