Flying empty planes round is madness.
It's an operational necessity, get real, they don't do it for sh*t and giggles, they do it to make sure ma, pa and the kids have a nice holiday.
Specifically, Glasgow has a traditional holiday called the "Glasgow Fair" which used to see extra Air 2000 and Britannia aircraft join the based units at GLA over a short period, indeed when Virgin launched service to GLA this was exactly the regime that was introduced.
If you want to serve that massive spike in the market, then you factor that price into the positioning flights or allow the other guy to fly those customers. In the trading cycle, we see years of UK based positioners and other years of the likes of <insert foreign carrier> and their ilk flying empty in from abroadland at the start and out empty at the end of the seasonal spike.
Two broad options :
1) Make the travellers take the bus / train to MAN to join the greater volume and freq there
2) See above !!!
The first one leaves the family knackered and frequently a tiring and grumpy end to a good holiday.