Whereabouts are you in Kent, M700? I'm in Tenterden, near Ashford.
I can only echo what the others have said, but in your case, I'd have the new Avon springs fitted and try to get to an idea from your mechanic why he's struggling with them.
I would also advise the fitment of some adjustable height 'trumpets', as below-
https://www.minispar...assic/HILO.aspx
These are designed to adjust the car's ride height and although you'll have nice fresh springs on the there, it's nice to be able to tweak the ride height to your liking. Personally, on a road car I like a standard/ high-ish ride height for comfort and to spare the car from impact damage from our awful road surfaces as much as I can.
Even on my bog-standard Minis I have Hi-Los. It also makes the front suspension easier to work on.
You don't need to change your rear springs is they are still functioning properly. They have far less weight to deal with and are rarely as worn out/ compressed as the fronts. Again, a pair of Hi-Los at the rear will allow you to tailor the car to your requirements.