in my opinion a local independant garage or specialist will do a better job as, more parts are changed normally as you can specify and it would be cheaper. nothing stoping you buying the bits you want changing from seat and doing it that way just pay the labour or do it yourself.
Most services are a oil and filter + inspection. They charge around £ 200 at dealers, to me thats just rip off and in all honesty paying for the stamp.
Every year i would prefer to change oil oil filter pollen and air, and maybe plugs depending. For the money it costs for all those bits its worth it an extra 10 quid for a filter isnt too much to spend every service to make sure you have a clean filter, pollen etc. all these bits with genuine oil dont come to over £60.
Also the cambelt change is overpriced, its basically like doing an 8v car only 1 pulley and its not the hardest thing ive done a few in around an hour. But the dealers have more overheads to pay IMO. maybe this is why the price is so high.