If a plane needs to go in for maintenance and the fleet isn't large then there might not be a spare plane available.
I trained at cabair. It's very professionally run but very over-priced. For example they charge you landing fees in addition to the usual fees when all Elstree based aircraft are exempt from landing fees!
The instructor I ended up with was truly excellent. But I went up with 3 - 4 other guys for various reasons. A couple were truly awful, not much older than 21 with a virtually no instructing time. As long as you can find a good instructor and pre-book with him/her well in advance you should be fine.
Personally I would in retrospect have gone with Firecrest.
Overall though, Elstree is a good place to learn. Very busy GA field, fairly difficullt runway (undulating with a slope) and complicated circuit patterns for noise abatement. Add on that it is right on the edge of London's TMA and you certainly won't feel out of depth after you qualify and want to go explore other places.