Hello,
They're quite pricey but Coast do beautiful clothes, and their assistants tend to be really nice too. They do lots of beautiful shrugs, often with beading. One of those would be really lovely glam way to cover the tops of your arms if you wanted to, and you could wear it with all sorts afterwards.
Warehouse often do lovely occasion dresses, and also really flattering and gorgeous jersey dresses that you can style up or down. Normally forty or forty-five pounds ish.
Not sure how old you are or what your style is, but my mum has lots of luxurious clothes from a shop called Nitya, she loves them for any kind of event. They cut trousers, camisoles and dresses beautifully in really gorgeous colours and fabrics. Lots of tunics and scarfs. They have an almost ethnic style. Imagine what Judy Dench wears - like that! I think they stock from an 8 to a 22, something like that. Might be tricky to find a store, though!
For a summer wedding, I'd also say wedges or flats - heels sink in the grass! And a cardigan or big scarf to wrap round you if it gets chilly in the church.
Enjoy yourself - I love shopping for special outfits!