Plug in your details here and it will tell you:
http://www.staralliance.com/en/services/visa-and-health/ The database which the website taps into, called TIMATIC, is what the airlines also use to check your eligibility to travel.
National United Kingdom (GB)
Embarkation United Kingdom (GB)/Destination Pakistan (PK)
Passport required.
- Passports and/or passport replacing documents must be valid
on arrival.
Admission and Transit Restrictions:
- Admission refused to holders of emergency and temporary
passports.
Visa required, except for Holders of a Pakistan Origin Card
(POC) or a National Identity Card for Overseas Pakistanis
(NICOP), regardless of nationality, for an unlimited period of
stay.
Visa Issuance:
- Nationals of United Kingdom traveling on business, can
obtain a pre-arranged visa on arrival for a max. stay of 30
days, provided: For details, click here
- Passengers of Pakistani origin can obtain a temporary
landing permit for 72 hours, provided:
For details, click here
Additional Information:
- Extention of stay if possible; to be applied for at the
Ministry of Interior.
Warning:
- Visitors not holding return/onward ticket could be refused
entry.
- Exempt are holders of a Pakistan Origin Card (POC), holders
of a National Identity Card for Overseas Pakistanis
(NICOP), holders of passports containing a "Visa exempt"
stamp issued by Pakistan and passengers having dual
nationality and using Pakistan passport.
In short, it just needs to be valid on arrival and does not need a minimum of 6 months remaining in the same way that you need for e.g. Thailand/Malaysia/Singapore. Of course, in terms of common sense you would want at least one or two months left on it, but strictly speaking it doesn't even need that.
And on a general point, I'm not yet aware of any circumstances where an airline cares that your passport details have changed since providing them at booking. They will just enter the new information at check-in.