Regarding the purchasing of Rukka, the nearest to you would be J&S in Plymouth, or Infinity Motorcycles at Yeovil or Southampton. You'd probably need to ring them first to see if they had stock, if not they may get it in for you to try on.
Sports Bike Shop will also get the kit in for you to try on, as someone has mentioned, the nearest to you will be Bristol. You can also order from them and return it (I think for free) if it doesn't fit.
Lately, I've bought most of my kit from Motolegends. Living up North, it's over a 500 mile return journey for me to visit the shop in Guildford so I've used their free returns service in the past to order any kit, try it on at home, and return what doesn't fit. You have up to 12 months to return it (but less if it was in their sale, you also have to pay postage to return if it was a sale item).
I've always found them really good to deal with, they will also price match any price, less a further 10%, if you find a genuine offer for the same kit elsewhere. I recently bagged a pair of Daytona boots, that retail at £499.99, for £269.99 when I found another store had my size in for £299.99!
Recently, on a trip to a music festival in Wiltshire, my Mrs wanted to divert to visit RHS Wisley. Finding out Motolegends was only 10 miles away I booked an appointment for a Shoei Helmet personal fitting service, dropped my wife off at the RHS Wisley gate and went back and joined her/picked her up a couple of hours later.
All I can say is I received excellent service from all the staff, they fitted the Pinlock and a Cardo Packtalk-S to the helmet, set the Cardo up for me, and they even arranged to courier the goods home so I didn't have to cart them around in the car for the next week.
The shop is well worth a visit, but it is best to let them know you are coming and let them know what you want to try on.