The Intergovernmental Panel on Climate Change's Fourth Assessment Report includes 'high-end' projections for global temperature rise from an ensemble of climate models. These are based on close to business-as-usual Special Report on Emissions Scenarios (SRES) (for example, A2 and A1B).
Using these projections, our current best estimate for a high-end UK-wide average annual temperature increase by 2050 (relative to 1990) is approximately 1.75° C.
In general terms, there is relatively little differentiation in temperature projections at 2050, no matter what emissions scenario is chosen. However, later this century, the gap between high-end and low-end scenarios will widen.