Hi MichaelC,
Just about any garage can check your tyre for you and should charge around £20 to repair it if they can, repairing issues with the wheel could be a bit dearer depending on what needs doing to it. Main dealers can be more expensive than other garages, this is a simple repair or replacement that doesn't need taking to a main dealer if you don't want to.
Most garages can get most brands and sizes of tyre delivered to them in a few hours from their wholesaler, if you want a specific tyre then it may cost you more or take longer, I usually just ask what they can get cheap, sometimes you get lucky and they have something in stock on special offer.
I usually get Yokohama if available as this is what was on the car when new, they are mid-price and seem to last and handle as well as anything else, Michelin is considered the best by most people, also rated highly are Pirelli, Goodyear, Bridgestone, Continental and Dunlop, as you would imagine, the more premium the brand, the more it costs.
It recently cost me £20 to have a split valve replaced.
Good luck