Initial outlay for a more energy efficient unit will translate into less cost over the life of the unit.
The 80+ gold psu's are worth it imo. Right now Super Flower, and FSP make gold units in the sub 700 W range that are good for systems with a lower power need. If you can get your hands on one of those I would recommend it. Rosewell sells rebranded Super FLower units here in the US. The Super Flower label might be easier to find in the UK, I am not very familiar with what is available across the pond.
This is a useful link, and i like the yearly costs table. However lets break that down a little.
80PLUS bronze PSU costs $105 per year to run
80PLUS Gold PSU costs $99 per year to run
That's based on 24 hours per day, 7 days a week, 365 days per year. Providing the user knows how to shut-down, or even put their computer into standby mode, you can easily half that figure.
So $6 per year saved, comes down to $3 per year saved, if even that much. Then take into account exchange rate, so $3 into £ is roughly £2. Spending £30 more on a PSU with a Gold rating, is going to take 15 years to justify that extra cost. And I HIGHLY doubt that the PSU will carry anywhere close to this sort of lifetime if the user even wanted it to.
EDIT: The above math was based on 100w Power draw, so if we triple it, and that's being really generous because it will likely use alot less than 300w 95% of the time, minimum 5years to justify the cost. Still I believe far beyond the reach of it being "worthwhile".
I would personally go for the antec dispite its lower wattage - the rails provide more power and its probably higher build quality (although Corsair PSUs are renowned for their amazing quality, the Builder series is their lower budget series)