I just had a look at mine for you, the clamps don't go all the way, however I believe the wheel comes with a set of what I'm going to call 'chocks' to insert between the clamps and your surface to alleviate the potential issue you allude to. If it doesn't, the problem is easily solved by using your own 'chocks' perhaps with some rubber stickers on to keep things tight. Local hardware shop should have something like that, they're usually to stop kitchen doors slamming shut loudly, should work to grip to the glass desk under pressure. I'm pretty sure I got some 'chocks' in the box.
As far as being the best sub- £100 wheel, I believe so. I was seriously impressed with it coming from a DFP, and many have commented it's like a 'baby G25', in terms of force feedback and features perhaps. The actual wheel is nice and big, which is an improvement in feel over the DFP.
However, the second hand market is such that you may be able to source a G25 for around £100 or sometimes less. That is the better wheel by a good margin, but you must use it enough to justify the cost. I'd go as far as to say the majority of G25 buyers don't use them enough to justify the price tag, hence a large number on sale second hand.