Help me find a suitably formal dress

[DeliciousIrony]

I've been invited to attend a formal event with my boyfriend in the spring. It's at his old school (very posh) but I'm not exactly sure of the dress code...he says he will probably wear a tuxedo, but wasn't sure of whether girls would need to be in full-length dresses, or if shorter ones would be okay.

So I need a lovely new dress, preferably under £100. I am size 8-10, reasonable-sized bust, 5"4, and aged 23.
I need to have my shoulders covered, and wear opaque tights due to scarring...so a sleeveless dress is ok if it can look suitable and non-frumpy with some sort of shrug.

The three I've found are:
This one

This one

and

This in blue

I seem to be erring on the side of shorter dresses as they tend to be cheaper as I might have more opportunity to wear again, and I don't want to turn up looking like a bridesmaid if everyone else is wearing shorter dresses. Should I be looking at longer ones? And are these any good? Or are there any better ones out there?

Thanks!

First and third are lovely...at a push I'd go with the first one.

I like the first one the best. It's lovely.

I'd try and find out if it was long dress affair before committing myself to a purchase.

The first from me too.

I don't like lace, so the first and 3rd is out.

Thanks for the opinions...although I do really like the first dress, I have decided to go for a full-length one after being advised that this might be more appropriate by someone who has been before.

So I have ordered this, which is the long version of dress 3 in my OP in green.
(Think I snagged the last size 10 as well, phew).

Not sure how to accessorize though? Would silver shoes work?

Good choice. The other dresses we not formal enough to wear if the men are in tuxedos. Silver shoes would be perfect. It can be trickey to get the heel height right with a long dress. Its probably best to try the dress on with different heel heights in your wardrobe before looking for shoes.