You can find information about Jury service within the Safe Families Handbook
It states that if you are called for Jury Service / as a witness in court you should let your line manager know (who will inform HR). Safe Families will give you time off to attend but will not pay you during this time. Instead you can claim loss of earnings through the court.
It could be that you are not needed in court in the afternoon and so can come back to work half a day. Please keep your line manager and HR informed of when you do and don't work, and what you will be claiming as 'loss of earnings'.
There is some helpful information about what you can claim on the .gov website - https://www.gov.uk/jury-service/what-you-can-claim-if-youre-an-employee
The loss of earnings form (to be completed by staff member and Safe Families) can be found on here - https://www.gov.uk/government/publications/claim-a-loss-of-earnings-or-benefit-during-jury-service