as I see it..
you are stuck on O2 and you have a budget.
this limits you!
here is what I think your options are..
I dont think you should waste what $$ you have for a used low-tech phone. Because in Oct.. you will much more choices and freedom. The possibilities are more open to you.
as a stop gap .. for now..
1. try rooting it again.. so that you can get rid of some bloat.. use ROMs with less RAM usage... move more to SD.
2. find someone in area.. to root you phone for you. of course have them walk you through it.
3. delete more apps... you can always reload them, when you need them
DropBox: register for free space (we both get 500MB extra space): http://db.tt/sWYVl3j6