The +-15' is not British but a IFPZ procedure applied to the EOBT, thus your taxi must be in accord and your departure within ETOT and by any chance not earlier than EOBT -15'.
If you wonder why, that's 'cause CFMU process your flight according the time you give him and the system decide whether you need to be regulated or not, any time the schedule is not respected the system got ******.
My guess, maybe spanish require you to be airborne exactly at your ETOT .....
