Unless you travel from the airport you can not enter the secure area of a London airport. And that goes for most airports worldwide.
You can follow this link to find out whether you need a visa. (It also given in a comment by @mlc, and has filled out some information which is based on what is given in the question but might not be right. as it is filled out it gives that you need a visa.)
An option to meet is for your friend to book a journey leaving on an international journey from the same terminal (or an other and you switching terminals) at the time you are in that terminal. This depends on your friend being able to leave and re-enter the UK and having the money for an international travel.
Some people claim that you can book a refundable ticket, check in, pass through security, meet with someone, leave the airport without travel and claim the money from the ticket. I would not do this myself but you will find people claiming it is possible.