Binance has ceased GBP deposits and withdrawals, so even if you sell BTC at that high BTC/GBP rate, you won't be able to withdraw your GBP, and if you try to swap it for USDT, you will get less USDT than the GBP/USD rate would imply.
When fiat deposits or withdrawals are not possible, it leads to mispricings that look like triangular arbitrage opportunities, but you can't take them out, otherwise someone would have done it already.