During a PhD which is funded is it possible to transfer to a different institution? I might be moving emmigrating abroad so I am double minded if I should start my PhD or wait til I have moved.

Would your funding be through a research council? If so, I don't think it would be possible to transfer it to a foreign institution - funding is given to departments for them to in turn allocate the money to projects (not to indivdual students) that are high priority to them. Also, since the British tax payer is where the RC money comes from, you wouldn't be able to 'take the money abroad'.

If you are likely to be emigrating then you should wait before starting. In most cases the grant is made to your host university or to the academic concerned. The money can move with the academic if they relocate, but not with the student. Even if this were possible you would find it very difficult to find an appropriate new supervisor in your new home. A PhD is a very specialised thing.