Take the advice of the other person earlier on.
If you can get away really early in the morning, then try. a stop at haverfordwest, and across to Kilkenny. Haverfordwest (A/G) Kilkenny(A/G)[Grass Runway with up/down slope]
Then return via North Wales maybe.
Will be long well above your 300nm route but a challenge
Additionally the 300nm c/x does have nothing to do with the school at all. It is the candidates responsibility completely. Further more the CAA could possibly take exception to the fact that a school had a hand in your qualifier. Its your licence application. Whilst it has been suggested that certain Integrated schools actually vet all of your solo flying and many cases make the decisions for you, the point of this is to test your judgement and true ability not just in flying but planning etc.
You just have to have it before you ask the CAA to issue the licence, so long as its 300nm it makes absolutely no difference whatsoever where it was you flew to, so long as it had 2 full stop landings not inlcuding your original departure airport.