Hi Drisgirl
The Glasgow Airport website states that Icelandair is the only operator on that route. (I'll give you the link since it might help suggest alternative destinations if you decide that Iceland is too pricey):
http://www.glasgowairport.com/portal/page/Glas gow%5EGeneral%5EFlight+information%5EDestinati ons+and+airlines/
The 12th of April is Easter Sunday, so anything around that date is likely to be particularly expensive. There seems to be nothing within your window for less than �400 return.
I'm sorry, but if you want to go away cheaply you'll have to change your destination. Paris is a possibility (around �110 return departing on 4th April) or Berlin (around �100 return departing on 4th April). Both of those are with easyJet.
If you're prepared to fly from Prestwick there are loads more possibilities with Ryanair. (e.g. Girona for around �83 return, departing on 5th April ; Krakow for around �52 return, departing on 4th April ; Gothenburg for around �21 return, departing on 9th April).
Chris