At the risk of opening a can of worms... the Aerad booklet for LL page B3 states
"An illuminated red stop bar means STOP. Aircraft must not procede until the stop bar is extinguished or ATC permission received."
One might consider, therefore, that if an aircraft is given a conditional clearance and that condition has been fulfilled, then permission to proceed has been given and the aircraft can continue to taxi over a red stop bar.
How far any pilot takes this is up to the individual, but I would contend that if we always waited for the stop bar to come down before taxiing, things would slow up quite a bit at LL.
I don't have the books for the other airfields in the UK with similar lighting arrangements, but I suspect that either similar notes exist in their plates (or perhaps less advisably, people are simply doing what they would do at LL).
Happy to be corrected. As ever, this has no basis more reliable than my own meandering experience.
G W-H