As a broad rule it's the newer brands (giffgaff, voxi, IDnet, smarty and so on) that have the best online capabilities, but always know which physical network they use. I find Three has the best signal at my home address, but their online capability is very poor, so the household phones are all on IDnet who use Three's network. It's been years since I used Vodafone, but even now I remember their crap online service, and even worse telephone and chat customer service, but if the Voda signal is the best for places you want, then voxi are worth a thought...and so on, adapt to which physical networks give the coverage you need.
For best value you might want to make sure you're getting a SIM only deal, stick to 30 day rolling contracts so that you're never locked in, and when you next need a new handset, buy it outright through Amazon or eBay - there's some stonkingly good stuff available from brands like Xiaomi, Oppo, and RealMe. I'm on my third Xiaomi phone, all have been excellent. A trip to Techadvisor.com will give you the lowdown on suitable handsets if the idea seems attractive.
