This page emphasises the importance of checking with your UK indemnifiers about their cover for you when you travel abroad to work.
Your professional indemnity cover will entirely depend upon where you are going and what you are going to be doing. You would be best to approach the organisation providing professional indemnity cover to sort out what cover you will be provided with.
If you are travelling to the developing world then short periods of experience abroad will often be covered by your UK indemnity. Longer periods, on an entirely humanitarian basis, may be covered by a reduced cost "missionary" level of cover.
The arrangements are often agreed on a case-by-case approach so take the opportunity to discuss your arrangements personally with your protection society.