bro my advice is on your current scenario
Job luck in UK? No! ----> Come to Pakistan and try, UK is not the end.
Parents need you? Yes! ----> Come to Pakistan and serve them.
Lonely in UK? Yes! ----> Come to Pakistan.
Brother not helping?Yes! ----> Come to Pakistan
No social life in UK?Yes! ----> Come to Pakistan
bhai UK is not the end of world, you have a whole life in front of you to prove yourself. It just needs sometime. As far as the money invested by your parent on you is concerned, to bhai is say kaheen zyada paisay to woh ap par yahan pakistan may bhee kharch kar chukay hain.
Come and say to them, that I will get a good job in Pakistan and will serve you for life, and I am sure they will welcome you with open hearts.
Have some mercy on them dear, they are old, ill and alone. They need you, and no one can replace you.
If I would have been in your place, then I would have left everything as it is in UK and come back to PK by the next available flight. (infact, mai jata hee nahi agr mere parents akelay hotay)
Himmat karo, or parents ki khidmat karo, baqi Allah pay chor do. Insha Allah wo kuch milay ga jis ka socha bhe nahee tha.