However the Glasgow to London section of the journey might be far cheaper with Megabus. They've got 4 departures per day from Glasgow. Single fares (based upon travel on Friday 28 Jan) are currently being offered at either £1 or £5, depending upon the time of travel:
http://uk.megabus.com/default.aspx
For the London to Stansted section of the journey it might be cheapest to use easyBus. Most of their services on my sample date of 28 Jan are offered at just £2 but some services cost up to £8.25. NB: easyBus services leave from Baker Street, not Victoria Coach Station. You'd have to take a No 2 bus, from Victoria Rail station to Marylebone Road, to get between the two. (£2.20).
http://www.easybus.co.uk
On my specimen date of 28 Jan, here's how it might work out:
Depart Glasgow 0830, arrive Victoria Coach Station 1730 (£1)
Bus to Baker Street (£2.20)
easyBus at 1930 (which allows plenty of time to get across central London and possibly to find something to eat), arriving Stansted 2055 (£2)
Service 510 at 2108, arriving in Harlow at 2155 (£4.50).
TOTAL FARE = £9.70
Chris