It really depends on how busy it is at passports. I mostly fly into T2 and use the ePassport gates, it never took me more than 5 minutes from plane to landside, but I often see lots of people queuing towards the 'standard' border checks. That being said, I understand [from your other thread] that you are planning to pre-book a taxi to Luton (from T2): if you are worried about giving them a time, you really shouldn't as they (i.e. the taxi company) would normally check whether the plane has landed on-time and be in touch with the passenger (or alternatively it can be yourself if the passenger struggles with English) straight after. In a nutshell: you should give them the scheduled time of arrival and they will think about the rest.

G