If the pension was paid prior to you returning to the UK you will need to consider split year treatment.
If you qualify for split year then you only report any foreign income for the UK part of the year.
RDRM12000 - Residence: The SRT: Split year treatment
If you do not qualify then you will need to report all your foreign income to the UK
Tax on foreign income
The guidance at RDRM12150 below will help you work out if split year treatment applies.
RDRM12150 - Residence: The SRT: Split year treatment: Case 4: Starting to have a home in the UK only
If this applies and you do not need to declare the pension then any capital you transfer will only be taxed if any interest is generated once it is in your account.