The economy leisure pax at LGW and MAN are generally paying less for their flight than those at LHR, tend to be more price sensitive, and have fewer options to switch to an alternate airline. Thus, VS have greater market power with these passengers, compared to those at LHR who will happily pay big bucks for a flight and can easily switch to an alternate carrier.
Yes, it would be nice if pax flying from LGW on VS to the Caribbean could be given brand new planes fitted out with the latest entertainment kit, marvellous food, champagne and excellent customer service, but this all costs money, for which most of these pax are reluctant to pay.
An airline is meant to be run as a business, despite what the pax seem to think !