DH chose the MK sockets (with a sort of curved top) for our house, you can get them from Screwfix or electrical wholesalers. They are about as nice as you get in white plastic. Screwfix (now part of B&Q group?) seem to have gone a bit DIY but they aren't that bad.
have a look at the prices on a trade supplier like this
there will be one in your town, but it may be on an industrial estate down a muddy lane. They will have a few displays but not a showroom and will be very busy first thing and around 5pm
MK have a well-known name, but they have been trading on their reputation and reducing their quality for at least 20 years. They still look stylish but are not very good or easy to wire up. My own preference is for Crabtree or MEM but your electrician will have his own favourite and will know local suppliers. DIY sheds are not renowned for quality or value, they sell on price and flashy design.
You haven't really said what your looking for OP, eg white, chrome etc however we postponed our socket replacement project as we are waiting to begin a bigger extension but our electricians preferred socket right now are Hamilton Hartland if that helps. They look pretty decent from what I've seen and aren't bank breaking.
Thanks - am just after plain sockets and a couple of light switches - I though flat white metal but tbh I might just go for plastic. I normally want fancy wood or antique brass to match kitchen fittings but I think plain might be better!