Is it possible to work for a prospective Canadian employer before being employed by them?

Could it be done via a route of the future employer, sub contracting the work out to a British company or self employed individual?

What would the Canadian company have to declare at their end?

Would taxes have to be paid by the Canadian company at their end or by the British Contractor at this end. In other words, "declared income".

If you're still in the UK working then I don't see there would be a problem. You'd pay tax on income in the UK like anybody else. Even after all these years I still use the same graphic designer for all our artwork and branding that I did when we lived in the UK. He just bills me in Canada (no VAT) and I pay him from UK funds held by our company. If you are living in Canada then I imagine without the necessary visa/work permit you'd be working illegally - whatever arrangement you come up with.

Still in the UK for a while yet.

Future prospective employer is having to turn away work left, right and centre due to staffing issues. Awaiting LMO outcome.

They have asked if I could do some work for them while I'm still here in the UK, so just wanted to check legalities.

Would benefit us both anyway, they don't have to turn so much work away and i can start getting used to their sytems and working methods.

As far as I'm aware, they're effectively just a client to you, giving you work. As long as you invoice them and pay the appropriate taxes as an entity in the UK I don't see the problem. My designer guy uses us to say he has an international client base!

Once you get the LMO and appropriate work visas, then I would I imagine you would want to move over here and work for them directly.