Need something nice to wear to a formal dinner

[Redrosesarethebest]

I have been invited to attend a dinner at a posh hotel with my son (he has to wear a tux) there is a huge possibility that Prince Charles will be attending. I don't want to spend a huge amount on a dress that I will only wear once, £50 max, anybody help?
TIA

TK Maxx is great for that sort of thing…what colour/size are you after?
black short
black long
grey long
blue

I am size 8/10 depending on style and fit of the dress. I don't know whether a long dress is needed, says nothing on the invite but I am guessing that the fact that ds is in a tux that it is formal