I suppose the fact that Firecrest don't have new aircraft isn't necessarily a problem as long as they are well maintained?
The vast majority of flying schools use somewhat elderly aeroplanes (for that matter, so does a fair chunk of the Royal Air Force).
If aircraft maintenance is done properly, it is not an issue. The elderly but effective C152, C172 and PA28 are all excellent training aeroplanes, and the mainstay of flying schools the world over.
By and large, maintenance is always done okay to good in the UK, and unless there are specific concerns being raised anywhere, I wouldn't worry about that.
It is embarrassing that most people nowadays are learning to fly on aeroplanes older than they are, but it's only embarrassing, not unsafe. If it troubles you, take up microlighting where the flying school aeroplanes are usually much newer (but neither more, nor less, safe by and large).
G
(Hired an aeroplane from Firecrest a couple of times a year or so ago, found them a fairly typical - safe and competent but not exceptional - flying school.)