Thomas Cook have been doing this for years. It's even more apparent in October, when you get almost every aircraft in the Thomas Cook fleet operating out of Glasgow for one weekend and back into it the next. I think they had 13 flights from Glasgow to Tenerife on the Friday of that weekend last October, including a couple of 767s.
The situation is slightly better this summer, as they have more W-flights, which can easily be routed to Glasgow for 4-5 weeks during the Scottish peak. So instead of operating LGW-DLM-BOH-DLM-LGW, they simply operate LGW-DLM-GLA-DLM-LGW. The Gatwick based crew that operated the LGW-DLM-GLA sector would overnight in GLA on the Friday and operate GLA-AYT-LGW of a similar rotation on the Saturday. So only two sets of crew need to be transported between the bases.
As has been said, they've been shifting aircraft around like this for years. If the bean counters didn't approve it would have been stopped a long time ago.