Hello and welcome to EasyExpat.com.

First of all, I would recommend to be careful with any plan for this summer. With the Covid-19 pandemic, July is close and although some European countries are starting relaxing some confinement measures, we still don't know what will happen this summer in term of travels. It might be that some borders are still close and the European Union might decide the keep the borders of the Schengen area shut, which the UK is not part of. Therefore it might be impossible for non French residents to travel from Britain... Again, I have no idea, but I just want to emphasize that better be cautious than sorry.

More generally, in order to give some response to your question, I have found the information below.

Several options:
Prepaid prepaid travel money card: Guardian Money has done a very good article on it and they say:

"The obvious solution for young people – or anyone with standard bank cards – is a prepaid travel money card. They are quick and easy to get hold of – there are no credit checks – and once you have loaded them up from your UK bank account at a typical currency conversion fee of 1%-2%, they can be used abroad just like any other bank card – but (mostly) without hefty charges."​

According to the article, the best options are Caxton FX Black, Sainsbury’s Bank Cash Passport and WeSwap. All will let you load multiple currencies – up to 18 in the case of WeSwap – and each allows you to make fee-free purchases in shops, and free ATM withdrawals – provided you are getting out the currency you have loaded on.

UK main banks: Easy but expensive > to be use as back up card
UK banks (assuming you are in the UK as your profile says) have got bank accounts for youths, that can issue a payment card. For exampel children aged 11 and above can open a bank account with MyMoney Children's Bank Accounts at HSBC and get a Visa debit card that they can use to withdraw in any ATM up to £100 (as long as the child has money on his/her bank account). Same thing with BarclayPlus, where you can get a cash card or a debit card from 11 years old. And the same applies to other canks such as Llyods or Metro.
With the Visa card they all provide, children are able to pay abroad.
Unfortunately they will charge a Non-Sterling Transaction Fee (~3%) for using the debit card abroad.