Hi, I have one of these suits, and they're fantastic collectibles - very highly sought after.
Your best option is probably selling through a reputable specialist auction house like Prop Store, they have regular sales of movie props and crew items, and have sold several of these suits in the past. You would be paying commission out of any sale price, but they will sometimes reduce or waive fees for 'headline' items.
Other options would be selling through another auction house - maybe Vectis who specialise in collectibles, or one of the big names like Christies or Bonhams - or on eBay, but again you're paying commission, and it may not come to the attention of as many buyers as Prop Store.
You could also just offer it up for sale on Facebook, with no fees to pay, but that's a bit of a jungle.
Honestly, if it were me, I'd be speaking to Prop Store first - contact Stephen Lane in their London office and he'll be happy to assist and advise.
As for value, with an auction the market will set the price, but for a complete suit I wouldn't be surprised to see it fetch some way upwards of £10,000.
Good luck with the sale!