I've access to, and dealt with, pretty much every alarm in the UK.
My personal preference would be Clifford. I like the features you can only get with the Cliffords, like BlackJax and programmable siren tones and easy set up Proximity sensor.
But I would also recommend Toad very highly, (and Stirling which is the same company ), I'd also say that Laserline, Sigma and Autowatch are very reliable, and worth considering.
As for where to get it done. You're choice entirely, but I certainly DON'T recommend mobile 'installers' like Gap. I've rectified far too many man-in-van, "but it was cheap", type installations. Go to a reputable car security dealer. If something goes wrong, you can simply take it back to the dealer. They'll always be in the same place
Of course, I should be recommending Sextons in Gloucester. If you want to get there, let me know as I may be able to get you a better deal.