It is some years now since I have stayed there but the boutique hotel Cranley at South Kensington is wonderfully located but relatively "quiet and secluded".
If you want some extra money to spend at Harrods, just up the road, catch the tube from Heathrow to South Kensington Station which is only a short hop to the Cranley.
The web site is thecranley.com and I see they have a "Romance Package"
Romance at The Cranley
Enjoy an elegantly themed evening surround by the seductive splendour of The Cranley.
Welcome drinks on arrival, a bottle of champagne and a box of chocolates in your room. Later a glass of Champagne in the privacy of your room, to relax and unwind.
•Double rooms from £199.00
Rates are per room per night for 1 night stay.
Rate includes:
•A welcome drink upon arrival.
•A bottle of house champagne in your room.
•A box of luxury chocolates.
•Continental breakfast for two.
Rates are excluded of VAT. Valid all year round, based on double occupancy and subject to availability.
Price: From 199 GBP