TAG Data is not top secret, it's viewable here (link to the UK Govt TAG data book)
Historical and reference information on all the appraisal and modelling values referred to in the transport analysis guidance (TAG).
www.gov.uk
Future costs that transport scheme planners use will for fuel/electricity prices for road and rail are in table A1.3.7 in the spreadsheet linked from the webpage above.
You are correct, the base data in WebTAG contains a single future value for each year. Road and rail diesel resource costs settle to a nice, stable level after 2030, which of course is absolute guff as history shows relatively regular energy price shocks (down as well as up) since at least the 1970s. We've had 3 in the 2020s! (Covid 2020, Ukraine 2022 and Iran this year)
So this potential volatility, if it's considered at all, is being done within the model(s) for the scheme.
It's pretty obvious that energy price shocks will continue to happen, and this should remind us that the kind of models people build using TAG or any other data are tools to aid thinking and judgement. Unfortunately, senior leaders get tempted to use them as a substitute for thinking and judgement and trot out the modelled BCR or whatever as "the answer" - i.e. they are outsourcing their decision-making to a spreadsheet, which is not what you really should pay people the big bucks for.