Just a few additional points:

You can only upgrade by one cabin when using Avios (points), so no chance of business even if you had sufficient Avios
Only certain, higher economy fares are upgradeable. Do you have an expensive, flexible fare already ?
You could buy the additional 2500 Avios, but you'd need to sure that award seats were definitely available. This would also be pretty poor value.
You can't upgrade at the airport using points.

I'd get to the airport early and see if they are offering any cash upgrades, as suggested by Stez.