Leo,
As discussed last night when I saw the car, I think the easiest and quickest solution is to adapt what you've already got.
You already have a 'Coilover' conversion on all four corners ( slip-over threaded sleeves, welded to the original strut tubes ) with 2.25in springs, although the shock absorbers are cartridge type inserts of unknown make / rate. You're already half way there...
Your main problem is that the car has run out of suspension travel at your desired ride height. If you raised it up by adjusting the spring seats you'd get back all the suspension travel that you need, but the car would sit higher than you want it to.
I think the best thing to do is simply section the strut tubes ( ie cut a section out of the tube and then weld it back together ) BELOW the threaded collars, and to install SHORTER shock cartridge inserts. That will give you back the suspension travel that you need at the ride height that you like. One of the most common conversions is to use a FRONT cartridge insert in the REAR, and to use a much shorter cartridge insert ( EG Toyota MR2 ) in the FRONT. There's lots of different options out there, but you might want to consider going to a top-adjustable cartridge insert like the Tokico 'Illumina' for example. 2.25in springs are as cheap as chips, and you can decide what rates / lengths you want to use once you have taken apart what you already have ( spring rate and length should be engraved on the top of the spring at the flat seat area ). You might even find that your original springs will be fine once you have shortened the strut tubes....
At the moment, your car is basically riding on its bump stops. In fact, it's actually sitting on the bump stops at the front even when it's not moving. Once you get the suspension travel back it will be MUCH nicer to drive.
The other alternative is to take off all four suspension legs and give them to a company like GAZ, who will convert them to fully adjustable legs for you. However, that's going to cost a few quid and you've basically already got half of what they are going to be giving to you.
If it was me, I'd shorten the strut tubes and fit Tokico Illumina inserts in them. Job done.