I’ve been in wearing chino shorts, t-shirt/polo shirt and trainers or Tom’s (slip on canvas shoes) without so much as a raised eyebrow. The wording on the lounge letter is something like “guests are asked to try not to wear flip flops or sportswear to the lounges”. Pool lounge is a more relaxed atmosphere and you’d probably get away with more there than 31st. Regarding tipping, I saw some people trying to tip lounge staff and they physically backed away from the money, very deliberately. The tippers had to practically chase them with the money. I think they had sorted something for them, rather than just served them drinks.