Is my smart meter on BST or GMT?

This is speculation on my part, but based on personal experience.

The peak/off-peak hours are set by the Distribution Network Operator (DNO), who know best how to balance their load. The timings are GMT all year round. 

Some smart meters compensate for this when the clocks change, while others don’t. Some time switches powering off-peak equipment (e.g. storage heaters and immersion water heaters) also compensate, but it may take a manual adjustment to the timer clock to show the same time as other clocks in the house. It usually makes no real difference either way, unless you find that a heater is switching on at midnight when the cheap rate doesn’t start until 1AM (if this does happen, then the ON period should perhaps be adjusted to exclude the first and last hours of the advertised period).

It does however create a headache for those responsible for the usage and billing pages of the online account. This is my usage table for 30 October 2022:
  

The extra and missing hours make it look strange, but even so, there are only 23 hours’ worth of readings. Stranger still, there are only 23 hours recorded for 31 March the following year. As far as I can tell, billing comes out right in the end. I’m not charged for an hour’s worth of heating at the higher rate when I shouldn’t be.

So, it certainly looks as if the timings for me are GMT all year round. You’d better check first what the hours are for your region:
  

“… use this table below; the first two digits of your MPAN is the ‘Area ID’; the ‘Off peak’ times are on the column second from the right:

Then check physically whether the meter is recording peak- or off-peak-usage during the first or last hour of night-time according to the clock in your kitchen. Make allowances accordingly!