if you are flying ba all the way, you should receive a boarding pass for lhr-lba when you check in at changi (ba does not fly to kl)....both ba heathrow arrival and departure will be at terminal 5....

you will need to clear immigration at lhr....that may take some time, depending on the crowds, but from experience, mostly can get through within 20-30min maximum....as a first time student, they may want to check your cxr, and that may take up some more time....then you need to clear security, which again depends on the queue...so 2 hrs would appear adequate but tight transit...

unless there is a delay coming in from changi, but ba will put you on the next flight to leeds...they have 3 flights a day to leeds from lhr...

alternatively, ask the travel agent to put you on the next lba flight anyway, which will give you a comfortable >6hrs transit....