I am 13 years old and have an accompanied visa with my aunt.
Can I enter the UK with my aunt and then return back to Pakistan alone (without her or with any other adult) if my aunts name is on MY visa?
I will readily presume your family is OK with this idea. It is not our place to speculate on that.
Despite what @GayotFow indicates I can't find any gender specific information. The only airline flying London-Islamabad direct, Pakistan Airlines Conditions of Carriage says:
7.7.1 You will not be accepted for carriage if you are under 5 years of age on the date the flight commences unless you are accompanied by an escort aged 18 years or more. If you are between 5 and 16 years of age, we can arrange for an escort, provided we receive 7 Days or more notice prior to travel and an extra charge is paid.
Nothing else in the CoC is about this age range, 7.7.2 only applies to 5-12 year olds.
I would strongly recommend to arrange such an escort. If something goes sideways, you don't want a 13 year old to deal with it alone.
Another airline which specifically mentions Pakistan is Jet Airways but only in regards of being a short haul flight for them, no gender specific restriction.
In general, without knowing the specific airline, the above is typical: above 12 the UAM service is optional but strongly recommended.