are you able to dismantle the boot interior to gain access to the latch mechanism and then release it from there?
Once you have released the boot then it would be wise to replace the latch mechanism which may be the faulty component
Here is a link to ebay for replacement handbrake buttons
https://www.ebay.co.uk/sch/i.html?_nkw=vauxhall+crossland+x+handbrake+button&_sacat=0&_from=R40&_trksid=p2334524.m570.l1311&_odkw=vauxhall+crossland+x&_osacat=0
Wonder if its a simple fault to resolve to get the roof working again?
If not then maybe look through fuses or find the chime generator and disconnect that
I would try to acquire an electrical workshop manual for the car as from memory I cannot recall any of the component locations so maybe best to look at a wiring diagram.
it just probably needs using and running at higher engine revs to bring the engine to temperature where the piston rings can seal a bit better - usually gummed up with carbon