This outfit has the rebuild kit for the MK2B:
https://jlspares.co.uk/sealkits.htm
The rebuild kit for the 2A will work, with caveats. Cut the band with a cutoff tool or hacksaw. I welded a couple of 1/4" (or 5/16"--I can't remember) fine nuts on either end of the band, then drilled one out so a screw (or bolt) could go through and engage the nut on the other side. The 2B rebuild kit has something similar already fabricated.
Another thing the 2A kit won't have is the 90deg rubber elbow that connects the switched air/vacuum pipe to the vacuum canister. Mine disintegrated on reassembly, but if you can reuse yours the rest of the MK2 kit is a fit (I assume they expect the elbow to be trashed, that's why they include it in the kit). The 2B kit also has a replacement rubber diaphragm, but mine was still in good shape. Also, if your servo has a lot of miles expect to have to sleeve the hydraulic cylinders (if you send the body out for sleeving tell them NOT to media blast around the vacuum/air switch--it's a machined surface).
With the exchange rate currently a little better for us Yanks, I'd opt for the rebuild kit. There's a manual on this page:
https://tinyurl.com/28dpmf Unfortunately, there's a couple pages missing in the PDF, but note where the small ball bearing goes. This is a check valve that I believe helps to eliminate the "thunk" that the 2A servo makes when you apply the brakes.
Side note: quite a few people throw in the towel and put an aftermarket servo in there (it may be cheaper when all's said and done). I have no problem with that, but I found rebuilding my MK2A--I bought the 2B for a spare--to be a very satisfying job (the known problem with the 2A is the dry lubricant coating on the inside of the vacuum canister--not an issue with the 2B). If you've done brake work before you can do this; study the manual so you understand thoroughly the operation of the servo (they're really quite clever).