Timers that use freeview AR (accurate recording) will be fine. These are the timers that have been set from the guide without any padding or subsequent editing.
The reason that AR recordings are fine is that underlying timer uses GMT all year round. It is only on displaying the schedule that a conversion to BST is done for the ‘BST’ timers. It is the same with the details of the guide where the underlying programme events are all specified using GMT but it converted for display purposes only.
I am not aware whether or not there are any potential issues with manually set timers. On the HDR-FOX T2 where I have manually set timers I also have a very short reminder timer set for the early hours of the morning and this would update the display clock before any of my manual timers run.