Easiest - would be to signup with Coinbase (referral link at https://www.coinbase.com/join/516b06afa03c8865ea000073 for free $10 of bitcoin if you buy $100) and then transfer your Bitcoin into it and then out to your Monzo account.
However, Coinbase don’t offer the ‘best’ exchange rates and do have some fees (0.0251 BTC at Coinbase’s current £5009.96/BTC and the £2.99 charge would get you £122.76).
I actually don’t know of any other trustworthy exchanges though.
Revolut do support cryptocurrencies and 0.0251 BTC at their current price of £4929.920/BTC would get you £123.74 (but their prices seem to be changing by the second: I can’t type fast enough!). So lower exchange rate, but no (obvious) fees.