In the US, the complete new servo (brake booster) is not that expensive, but of course, it comes down to replacing it on the RHD cars - engine lowered, etc. There's two part numbers so check the years.
https://www.fcpeuro.com/products/vol...649491#fitment
https://www.fcpeuro.com/products/vol...93673a#fitment
For the URO seal alone, to be fair I think URO is insane asking $40 for a piece of rubber. When I bought mine it was somewhere around $20. They know they don't have competition. Search on Ebay "Seal URO 0014300708S" As for installing it with the servo still inside the car, I think it's a very fiddly job and several people commented on the video they couldn't do it. I understand having the clutch pedal in the way makes things even worse. One must absolutely use that piece of cardboard to push the seal sideways while pressing it down with the fingers, without it it's impossible to do it. The other thing is to toss the plunger as needed sideways to increase the gap at the place where the seal is being inserted - otherwise this gap can stay too small and the seal doesn't go in. There a big comment with these tips in comments section.