https://openledger.info/ had fiat gateways for USD, EUR, CNY but they were temporally closed. Normally I just buy Dash on Cex.io and transfer to wherever I trade (mainly Binance) Dash always confirms in 15mins and it's super cheap. CEX.io supports GBP. I've also heard many good things about http://uphold.com/ from Dash community. Try those two for the time being (or try an ATM if it is available)