friends posh birthday lunch, what to wear?

[SoupKitchen]

My friend is having a birthday lunch in 3 weeks time at the poshest resteraunt in town.
Lunch is 12.30 on a sunday.
The restraunt is in a busy seaside tourist area that will be heaving for the BH
I will have to walk a little of the way there.

What do I wear, any suggestions BTW due to the expensive lunch I have a limited budget, £50 for outfit.

BTW am size 10-12,
broad shoulders small saggy boobs
big hips

The top and tunic looks nice, I never do any thing posh these days so am totally out of touch.
I am blond with fair skin would that outfit still work?

thanks tmmj

See this from another thread...I am going to try it at lunchtime.

topshop orange dress