There is no specific tax relief for VFX or post production in the UK, but both are allowable costs within the Audio Visual Expenditure Credit (AVEC), providing the qualifying criteria are met.
You do not need to perform all production activity in the UK and it is absolutely possible to claim UK tax relief for post and VFX only, even if no principal photography occurs in the UK.
The current rate of relief is equivalent to 25.5% net after tax. A minimum UK expentiture requirement of 10% of total budget must be exceeded, and the project must qualify as British via the Cultural Test or be an official co-production. Note that there may be sufficient points available in the Cultural Test to qualify if only performing VFX or post in the UK.
It is essential to set up a UK based limited production company (FPC or TPC) before shooting begins in order to account for all production costs, even if only the post-production or VFX occurs in the UK.
Currently UK tax relief is available on up to 80% of the total production budget. However the UK government in their Spring Budget 2024, proposed that VFX expenditure incurred from April 2025 will be immune from the 80% cap on eligable expenditure, and will attract a higher rate of relief of 29.25% net. This will mean that productions could receive 25.5% relief on filming in the UK (on up to 80% of total budget) and still receive a 29.25% rebate on their VFX spend regardless of whether that spend takes their total UK spend above the 80% cap on eligability. [Note that due to the general election in the UK, we are awaiting confirmation from the new incoming government that they will honour this commitment.]