I'm a software developer, fresh graduate and my first job after grad is 3.5k(3k post tax) and I have been working close to 6 months since then.

Now my boss want me to shift to another UK based company while working part time for my current company, which puts around 5.25k(4.8k post tax).

I definitely think you are underpaid, 8 years experience is damn a lot especially if you got quite a lot of them in mobile development(higher demand).