Hi, and welcome to the forum.

1. Easier than what? Technically they should accept any EU degree as valid in the UK, but from the employer's point of view, a UK degree is easier to understand (i.e. if they are hiring PhDs in polymer tech, they probably know exactly what that means if it comes from a UK university - for Spain, they have a good idea, but the programs could differ).

2. n/a

3. What profession are you looking to enter? In many countries it's assumed that a PhD means that you are looking for a university level teaching job. But in the UK (and other places) you could also find a job in pure research (i.e. a government job with civil service status) or in applied research (i.e. with a big, international company) or with a small start-up company. The salary ranges vary greatly.
Cheers,
Bev