If doctors wish to move within the UK, they must apply for an inter-deanery transfer, but there are limitations on how this can be used, and if it will be approved. It is not considered a right.
Trainees may take breaks from training to work outside of the UK, but there is a limit on how long this can be done for (maximum two years).
Generally, it is not possible to accredit training opportunities outside of the UK for the purposes of having this taken into account. (also some out of programme training opportunities do exist) There is no guidance available on whether it is possible to move into UK training part-way through a programme if a similar programme has been started outside of the UK (or vice versa), however this can be expected to be very unlikely.
Where else can we find more information?
https://www.bma.org.uk/advice/career/going-abroad/working-abroad/returni...
https://specialtytraining.hee.nhs.uk/nationalIDT
https://www.copmed.org.uk/images/docs/gold_guide_7th_edition/The_Gold_Gu... (pp.36-43)