Can anyone tell me what is the minimum salary I would have to earn for tax purposes if I was to become 'employed' by a company? And roughly how much tax I would need to pay? I heard it was different for different nationalities. Im from UK and was told 50k a month is the minimum outside of teaching?

There is no minimum salary to get a work permit.

The salary requirement for getting an extension of stay at immigration under clause 2.1 of the police order is 50k baht for a UK national.

If you cannot qualify for the extension you can get a multiple entry non-b visa in Penang if you have a work permit with at least 8 months of validity remaining.