In my experience, third-party warranties aren't worth the paper they are written on. There's so much small-print and clauses, and in some cases for a mechanical issue claim for example, the car has to be serviced every 6 months in order to comply and be stamped by VAT registered garage. That was my experience with an Auto Protect warranty.
Not sure about Group1 Assured. Bear in mind, car is 2014, so now 12 years old. Check condition of roof, and carpets for any water ingress. Also check roof function, is it smooth operation, or any hesitation? Any issues with the roof - then flag with garage before the deal is done. Again, any roof issues down the line, avoid the main dealer or you'll be crippled by their labour rate. There are third-party businesses that specialise with convertibles, that will do a better job, even if it means travelling and a couple of nights accommodation.
Example here: https://hoodtech.co.uk/audi-a3-cabriolet-2013-2021
I think with an older car - set aside a fix-fund for a potential roof fix, and hope it never happens, and service the car in line with manufacturer recommendation.
Regards S Tronic transmission. This needs to have been serviced every 38k miles, no exception.