1KDAN;
Heathrow does not, to my knowledge, offer flu shots to travellers. We've got enough to do to immunise our population at risk (elderly, asthmatic, etc..)
The only clinics I know of is the British Airways travel Clinic at Picadilly:
http://www.britishairways.com/travel...o/public/en_gb
They do offer flu jabs and the cost is £25 (about $50) and they say in their list "for travel purposes only" (whatever that means).
BTW, UA arrives at Terminal 3.