A lot of bikepacking events require a tracker. I use the SpotX, which allows you to pay for a small annual subscription, then pay by month for when you actually need to use it. As my need for it is sporadic through the year, this works out quite reasonably priced.
Even so, if you are just intending to ride a single event in the UK, a Spot is perhaps overkill. But you should look at the underlying need, which is presumably to ensure somebody will know if you’ve hit a problem and are unable to move or raise an alarm, and to get emergency services to your location. That could be fulfilled by other riders on the course.
Other options would be to rent a Spot as proposed by Phil, or borrow one if you know somebody with one.