How to get around not having a trading pair for a crypto I want to buy with my fiat (gbp)

Hi guys newbie here, been using coinbase to buy crypto to hold but looking to avoid the mad fees now.

I’ve now been introduced to trading pairs which is obviously not the case on standard coinbase. I want to buy some Polygon, but can’t buy direct with gbp on binance. Is there a process for buying day btc and then purchasing crypto with btc as opposed to gbp? If so is there a crypto that is fairly consistent so I don’t lose out on exchange rates while making the trade.

Thanks