Wow! You are all amazing. Thank you so much for everyone's suggestions.
I shall browse through properly when I get some undisturbed peace this afternoon, and update you then.
To answer a few quick points...
(London) Greek Orthodox ceremony, so its religion and etiquette totally unfamiliar to me.
But both families are devout, so I'm respectful of that.
No mention of head coverings. Bit concerned now. Will double check.
The dress was a big investment on my current budget, but I know it will definitely get a lot of wear for years to come. Hence remaining £40 has to be the absolute max for a cover-up. Or rental cost (thanks for that suggestion...hadn't even occurred to me).
Yes, already asked suitable candidate friends re borrowing something suitable, but August being peak holiday/events month, they'll either be in use or are wrong size/colour (or so fancy I'd worry about my tendency for spilling red wine! Hmm. Maybe that prevents hire possibility too?)
Really appreciate your help. Will let you know what I decide upon.