Best options for changing British pounds to Euros for France?

Maybe you’ll come across one of those ATMs in the UK that dispense Pounds or Euros. I haven’t come across many of them and never got money from one.

I can't imagine the dynamics with a UK debit card are any different than a US debit card, if so, then yes you can get euros from an ATM in France, and that would be your best deal. The conversion rate should be determined by the card processor and about 1% of the transaction. Of course your bank may charge a fee for the transaction, and the ATM you use may as well (a few euro), but many ATMs in France (unless it has changed) do not charge fees. Do decline any DCC offer, do the transaction in euros and decline any conversion offer.

As for currency speculation, that would just be pure gambling. I am not sure you could say whether the euro or GBP will be more, or less, affected by the war.

Gbp has moved one cent against the euro in the last month?

In any case you are unlikely to need cash in either country. I haven't used cash in almost a decade in the UK. Just get a small amount of the local currency from an atm in each country when you arrive, maybe 40gbp/eur for emergencies.

I agree that you're not likely to need much cash. I usually get the equivalent of around $50 (around £38) at ATMs once I arrive, and I almost always have money left over because I pay for everything with credit cards, using Apple Wallet.

When I do need money, I find an ATM that's physically connected to a bank. You can always check the conversion rate using your phone. If you have an iPhone, the default calculator is able to convert money, or you can just download a conversion app.

I can't imagine you'll need much cash. I haven't used sterling or euro cash since early 2020. If you want to carry, say, €50, you could just withdraw that from a french ATM. But mostly your best bet is a credit card with no foreign transaction fee (like Nationwide or Halifax Clarity). If you prefer to buy euros before you go, you could use a prepaid card like FairFx or Caxton FX, which will offer a better rate than buying cash from the bank, Tesco, M&S, etc.

My opinion, cash in pocket is like a spare time. You will probably not need it, but heaven help you if you do and you don't have one.

Although I'm generally cash-phobic, I have to say there's a more compelling case to be made for carrying it in rural France than in the UK. So many small-town merchants in France would be harshly penalized by card use for small purchases. It's quite easy to find ATMs in France, of course.