Here is the official policy:
Strollers: One fully collapsible umbrella-style stroller may be carried onboard, in addition to the carry-on baggage allowance. Strollers are only allowed on board the aircraft if approved storage space is available.
Northwest passengers traveling with children may bring their stroller to the gate to be loaded there. A pink stroller tag will be attached to the stroller. At arrival, strollers tagged with pink tags will be brought into the boarding bridge for passenger pick up. *Subject to airport security policy.
It specifically says that it is for passengers traveling with children. I guess the question is what would happen if they find out you don't have a child? Could you check it at that point as excess baggage? If so, it might still be cheaper than purchasing in the UK.