You can accept Euros and have it go into a Euros balance in PayPal. Fees are the usual if you accept it as a business/trade transaction. None if marked as gift (with lack of protection obviously).
If you then pay for something in Euros from that balance then no conversion fees.
If you pay in GBP you’ll be hit with PayPal’s high conversion rate.
I would assume if you accept Euros as GBP you’ll get hit with the conversion rate also. The rate is usually much higher than credit cards.
I’ve got a bunch of Euros in PayPal just sitting there waiting for me to use them on something. Problem is it’s a small amount. If I buy something costing more then PayPal will charge me a lot to convert GBP to Euros for the remaining balance. Although I think I can insist they don’t convert and charge Euros to my credit card which may be cheaper.