I use Momentum99 which is Tesco's premium unleaded. It's the same octane rating as V-power and whenever I've been on the dyno I've always been pleased with results.
I used to use V-power, but the reason for swapping to Momentum99 was because not only was it a few pence cheaper, but tesco clubcard rewards are much better than the shell driver's club, which send you a petty £3.50 voucher once a year!