interesting that this comment has been voted up :) - you are not the target market - if you can spend the cryto you HODLing on Monzo as easily then your comment is valid.
The amount of KYC/AML nonsense you have to go through with Coinbase UK is higher than Monzo/Revolut/Transferwise/Starling, so if you qualify for Coinbase UK card you qualify for any number of better solutions. If you refer to the conversion of crypto to fiat, you simply withdraw to your Monzo or whatever and spend, yes it's an extra click but it beats the embarrassment of card rejected as acceptance of these clown cards is very hit and miss (despite the Visa/MC network backing).
The problem with most banks in the UK is they'll usually lock or even cancel your account if you're dealing with Crypto. Monzo is well known for doing it (Google "Monzo closed account bitcoin").
Barclays is one of the only banks slowly accepting Crypto related transactions.
Most people I know dealing with Crypto are using Revolut as they are not a proper bank. However, they recently blocked transactions to Coinbase's account (this is most likely because they are applying for a UK banking license).
But then you are just not the audience; people with crypto can already (in some countries) convert to fiat indeed, but they do not want to; they want to pay 1 coffee and 20 minutes later a sandwich etc and leave the remainder in crypto. They do not want to conciously get up and convert 100 pounds, do their shopping and repeat that the next day.