I redeemed Avios points for this experience. The venue is very luxurious. I did feel a little out of place when a staff member took my down jacket. It was a very cold day so I had to dress warmly, but my impression was that fancy dress would be preferred.
Servers were prompt and efficient. The afternoon tea provided a great selection of sandwiches and fancies. Leftovers were boxed so I enjoyed these later in the evening at my hotel.