Wow that's cheap!
National: I wouldn't be bothered about even entertaining a trip to kwik fit for new tyre for the Coupe - as the MOT has lapsed it'll be four loose wheels in the boot of the C5, so their options for "up selling" a set of brake pads or whatever would be severely curtailed!
I've used BlackCircles and MyTyres in the past, both have links with garages very local to me so convenient for that job.
In terms of tyre brands I tend to default to Uniroyal as a good wet weather tyre at middle of the road sort of price. Rainsport 2, and 3. The 2 l Coupe has RainExpert as they have deemed no one with 205/60R15 drives in a spirited manner!
The C5 had a new pair of Goodyear efficient grip performance 2 which I would deem "fine" was bother about the Uniroyals and their soft side walls on such a big heavy car. Have since read on here lots of folks don't like these goodyear things, but also tyres generally seems to be a massive minefield of differing opinions!!