Bok choy is a cool season crop and bolting is triggered by lengthening photoperiods ( daylight hours exceeding 16 hours, or darkness less than 8 hours for a month ), and temperatures below 55 deg F ( 12 Deg C ). Once bolting has been triggered, the plant can not return to the vegetative stage. Short days with warm temperatures will prolong the vegetative stage.
Therefore unless you can artificially control lighting you should ensure your plants mature before they reach the critical photoperiods. This can be accomplished by planting several weeks after the summer solstice so that it meets lengthening hours of darkness.
Since your plants are bolting in Scotland in late March, when there are only 13 daylight hours that would suggest your problem was triggered by cool temperatures. The average temperature in March is 6 deg C, and in an unheated glasshouse may only be a few degrees more than that, and not enough to stop the bolting.
Edit: 24 May 2020
The author Joy Larkcom says Bok Choy needs to be germinated and grown at 18-22 C in its first month otherwise it bolts. Thereafter it tolerates colder temperatures. She also quotes a Rodale study that showed that Bok Choy grown at an average 10C bolted.
How to stop spring crops bolting
Specialty and Minor Crops Handbook, 2nd Edition by Claudia Myers
Oriental vegetables, Joy Larkcom 1991