For 2,500 GBP you currently get the most USDT on Revolut X: 3,331.53 USDT. The least favourable exchange (Kanga Global) gives you the equivalent of 27.73 GBP (1.11%) less.
Exchanges with a direct USDT/GBP market: Revolut X, Kraken, Coinbase. Elsewhere you need a currency conversion or an intermediate currency, which usually adds cost.
Counting only the trading fee and slippage, OKX wins, but once deposit, conversion and withdrawal are included, Revolut X comes out cheapest.
Revolut X leads both for 250 GBP and for 25,000 GBP, so it also has the deepest market for this pair.
Even for 25,000 GBP, splitting the order across exchanges brings no benefit, so one exchange is enough.
GBP deposits: Faster Payments (Kraken, Coinbase, Bitstamp); Card (Coinbase); Revolut account (Revolut X).