TELL, the master "who knows" is back....
I read this as you have limited experience (if any real) with car hifi, so I can undestand that that you are so proud that you were able to install the infinity and autoleads parts in your Altea. Naturally, that have lead you to believe that this is the only right solution...
I have played a bit with the Leon 1P (had it since april this year...)and even with an Altea (have one as a company car - reg. as a van...).
In my Leon I used the Aluminum rings from Exact, and for the Altea we used a router to shape the rings, of MDF, due to the shape of the Pioneer speakers installed.
Blaupunkt is available in UK, just order them via your
Seat dealer...
Tell, my dear friend, please accept that there may be more solutions to problems than yours, and please also accept that other than yourself can suggest the solutions here in this forum.
PS. Can I get a better backup than SEAT???????????????