Boots.com says they do presciptions (the same services are listed for T5 landside departures, for reference)

Boots Store Finder:
Address
Terminal Three Landside Departures
Hounslow
Middlesex
TW6 1QG

Pharmacy services
(NHS) Seasonal flu vaccination service
(NHS) Stop smoking service
(Non-NHS) Emergency contraception
(Non-NHS) Emergency supply of medicines
(Non-NHS) Seasonal flu vaccination service (not at risk groups)
(Non-NHS): Travel clinic
Appointment booking available for consultations
Body Mass Index (BMI) Machine
Boots Macmillan Information Pharmacist
Electronic Prescription Service
Emergency Contraception
Hair-Retention Programme
Medicines Check Up
Medicines use review (MUR) service
Multi-lingual staff
New Medicine Service
Prescriptions Direct
Private consultation room
Repeat Prescription Service
Stop Smoking
Travel vaccinations & advice
Winter Flu Jab Service