As far as I know if you are planning to use Moonpay in the UK, you will have to get a valid ID.
After that:
How to use Open Banking to top up your MoonPay Balance.
support.moonpay.com
This page should put you on the right track. Step-by-step with screenshots.
One time step > link your bank account via Tink
Then just choose Top up from the menu.
Et voilà, now you can buy BTC. Count on having a holding period before you can transfer it out.
This is important:
*Also get a non custodial wallet. Transfer BTC from Moonpay over to this and from there you can pay your vendor. "Grey" payments from an exchange have a tendency to get you in trouble.
*For every step in this process you can send a small amount like £5 first, see what happens, then send the rest. Better safe than sorry.
PS: There is lots of information on crypto on this forum. Please have a look before you dive in.
I wish you all the best!