Wear whatever you want. We did the tea in May and I couldn't tell you what another soul in the place was wearing. Nobody, and I repeat nobody, will bother to look at what you are wearing and give it a second thought. We went straight from the MK, wearing shorts and a cotton top (me a polo and DW a collarless maternity shirt). Go, be comfortable, and enjoy!!!
We had a great time in May and even (gasp!) brought our 2 and 4 year olds. When our 2 yo DS woke from a short nap the waiter took him off our hands for a few minutes, walked him around the dining room and back to the kitchen to say hi to the staff (all the while never out of the sight of watchful mom and dad of course

). He loved is, the other guests thought he was the cutest thing and loved it..............people are just so into relaxing and having fun they don't really care what anyone else is doing.
Ok, so maybe you wouldn't want to go in a thong, with a crying child in tow - but other than that, don't worry.
Funny story. When we were at the tea the only 'uptight' people we saw was this big group of folks from England (vacation week in England you know). After snickering at the menu like a bunch of snooty, 'I'm too good for this pathetic American excuse for tea', English windbags they decided to slum it - only to be told there were no tables available. Ha!!! Served them right. Well, all of a sudden the tea was so good that they just had to be squeezed in

.