I have never fought a charge this serious before, but here are the points I see that the prosecution has to prove...
(1) Identify you
(2) as the DRIVER
(3) of a MOTOR VEHICLE (as defined by the HTA)
(4) that was on a HIGHWAY (as defined by the HTA)
(5) that you were in a RACE or a CONTEST
(6) and that you were performing a STUNT (or that you there was some kind of BET or WAGER involved).
So you ask a good question of "How can they interpret it as a race or contest and performing a stunt"!
I do not know the answer (yet), but what happens is other people have probably asked that same question and the come to court and challenged it and then it got appealed and some appeal court judge has decided that a certain something specifically "is" or "is not".
So my guess (only a guess right now) is that there are some case laws where a Judge (not a Justice of the Peace) has decided that speeding over a certain amount constitues a "stunt" and also constitutes "racing". If there are case laws that support this, then the prosecution does not have to do much but actually prove your speed and the case law sort of automatically proves the stunt/race part without much effort on the prosecutions part.
Now of course you can always try to challenge these decisions, but you have to have a really great argument AND you have to be really good at presenting the argument as well.