I have 1 year House Job experience and 5 years working internal medicine in Pakistan

Hi guys, I have 1 year House Job experience and 5 years working internal medicine in Pakistan. I got an offer letter under the MTI program. The offer was for 2 years contract with a salary of about 36,000 GBP. I signed it and sent it. Now I am having second thought that this may be a low salary. Can I negotiate something higher with the hospital now?
And what is a proper worthwhile salary for someone with my experience?

The hospital in question from which the offer came is somewhere in the north of england. I am not familiar with the area at all as I have never been to england.

Thanks.