Electrification to Newbury, Didcot and Oxford is due for completion by 2016 and that to Bristol and Cardiff approx 2 years later. Don't expect to see much visible indication such as masts being erected for some time yet. The most visible evidence will be in the rebuilding of bridges to allow OHLE underneath.
In Wales the wires are still planned to stop at Cardiff. I personally think that there will be a change of mind and electrification will be extended to Swansea (but who knows).
If you area asking about further electricifaction to the West of England, that's something for way into the future. Ask again in 10 years time. Network Rail have their hands fuil at the moment with the current plans and the probable Valley Lines electrification around 2018-20.
That is something I have been thinking about it would be better if the wires were extended beyond Newbury to say Westbury to cover the semi fast Bedwyn - London services which could be extended to/from Westbury if possible and stock is avilable.