They do not revoke transit visas on a whim. There needs to be a concrete reason like you obtained it fraudulently or you are using forged documents. The law says that a transit visa means the Home Office has already checked you out and you are cleared. From the rules...
The holder of such an entry clearance will not require leave to enter
on arrival in the United Kingdom and, for the purposes of these Rules,
will be treated as a person who has arrived in the United Kingdom with
leave to enter the United Kingdom which is in force but which was
given to him before his arrival.
An Immigration Officer's discretionary latitude in these cases is very slim, if any.
If your nervous behaviour sets off the IO's radar, they may pursue a more intrusive line of questioning. Even then if you have played it straight up, there is nothing to worry about.