If you are near Dorchester, Mud sweat and gears is a good honest shop, there is one in Weymouth and Dorchester. I’m not sure they would service your lefty in-house, but I’m sure they would arrange it all for you.
Also worth joining their club, it’s £5 per year and gets you 12% of all accessorise. Really good if you like endura, as it makes it cheaper than on-line.
Charlie is good, but doesn’t stock much, great for singlespeed bits though. Will point you at some awesome riding. Cyclexperience in Wareham is also ok, but you won’t get a discount, they do have good stock and wheel building though.
Primera in Bournemouth is also good.
The only rubbish shop was Dorchester Cycles, but that has now closed. Just avoid anywhere that also sells car parts!