However, in order to join I'd need to abandon my British Citizenship - which will be a massive decision to make.
It is not possible to "abandon" British citizenship. You can surrender your British passport (which may be what the Pakistan authorities require you to do) but you can't stop being British...
Should you wish to apply for another British passport later in life, you will be treated exactly the same as any other Brit who has not held a passport for several years. You will just need to provide relevant documents to prove British citizenship, and then you will be issued with a new passport.
I have four passports from four different countries - all legitimate. Sometimes I let my UK passport lapse if one of the others is current and more relevant to my current state of residence. If I subsequently need to renew my UK passport, it is very easy to do so - just send in a photocopy of the old one, up to date witnessed photos and a cheque.
As far as I know, the only developed country that allows its citizens to formally renounce citizenship through a legal process is the USA. Some US citizens choose to do so in order to avoid paying US taxes, which are levied on all US citizens wherever they may be resident.