I do the port dance with Lebara every six months or so and Vodafone (those 99p SIM cards in Tescos etc.) always do the trick.

I activate them, then register them (as I am paranoid about losing numbers) and then port in and back out. Have never had to recharge the sim first.

What I will say though is it has to the pay as you go sims that are in the supermarkets. I got clever once and ordered a free pay as you go plus sim from their website. As that one is an online only product, it did want me to set up the bundle before it’d let me do anything.

Ironically 3 were always another good option but again, no good for you as that’s who you’re with. O2 wouldn’t let me set up an online account or do number porting without topping up first and I’ve not been anywhere near EE for a long time so not sure how their prepaid sims work.