This is a question I get all the time from new clients.
The quick answer is £500 per month at a bare minimum and £1000 if possible. That’s of course an answer I give without having a clue about your business!
The longer answer is that you should spend whatever you feel comfortable spending on Google Ads. There is no minimum required. However, if you’ve paid someone to set up campaigns, there’s no point in having an ad budget of £10 a month. You will be getting very little traffic and it will have been a bit of a waste of money to pay someone to set up the campaigns only to severely restrict traffic to them.
If your campaigns are correctly set up and your website is converting the traffic, the more you spend, the more sales or leads you get and the more conversion data the account will get. This conversion data is very helpful to optimise the account and provide feedback to the bidding algorithm, so the more you spend, the better the account becomes and the more you can spend again because you’re making even more money.