EE is the only decent network with full coverage throughout the area, good speeds even in the middle of the city centre, and signal below ground in the metro stations and tunnels too.
EE commands a price premium for their network. But it is very much worth it.
Are you sure it makes sense to buy for the free music packages though? Are you sure you can’t get an Apple Music deal elsewhere or even pay for it yourself separately and save money overall? Even the likes of the Shazam app will give you 6 months free Apple Music the first time, then 5 months, then 3 months, then discounted rates after that.
The reason I ask is because if you don’t need loads of data especially, you can get onto the EE network for cheaper by going with a brand that piggybacks on their network. Plus no contract either.