It is indeed very possible to grow cacti in the UK, they won't grow as fast or as large as they would in ideal situations but they will grow quite easily.

If you want to speed things up you can use a grow light set up and/or a greenhouse in the warmer months. If it gets really cold it won't be so good for them to be outside.

Several of the Trichocereus, recently renamed Echinopsis, group cacti (pachanoi=San Pedro, bridgesii=achuma, etc) are high elevation cacti used to more cloud cover, cooler temps and higher precipitation than their low desert-based cousins, so UK should be fine as long as you bring them in during the winter. In fact, near sea level in southern California I actually sunburned some specimens this past summer...imagine: an entire 1/4 of the plant becomes pale white and presumably free of alkaloids.