I just researched this when buying a mobile router / MiFi device for our van. We’re heading to France soon for a couple of weeks.
I went with a BT Mobile SIM - unlimited data, £20 per month. BT Mobile is effectively EE in the UK as they own the network. However BT Mobile still offers free EU roaming on their plans, unlike EE. With a “fair use” cap when roaming of 50GB per month which is decent. In France they use the Orange network. I took out a contract as I’ll use it in the van in the UK too.
Other options I found that still offer free EU roaming are Lebara (uses Vodafone), Tesco Mobile (uses O2) and Smarty (uses 3). Check the “fair usage” policies though - for example, Smarty caps data at 12GB per month when roaming abroad. If I didn’t want to contract I’d have gone with Lebara, they offer decent pay as you go SIMs with plenty of data for free roaming in the EU.