If it is a newer gun with the round tang rear sight, an LPA sight mounts using the factory D&T'd holes in the top strap. S&W uses these on the "Competitor" series.
Here's their US distributor's website...
Precision Sales International, Inc.
Here's LPA's website so you can find a distributor in the UK...
http://www.lpasights.com/en
It appears they are trying to say they can be used on the older guns (square tang rear sight) also, but would require you to have the frame D&T'd and some possible milling of the sight channel.
Bowen Rough Country sights are another alternative. Pricey and hard to find (well, easy to find, hard to find in stock).
I have the LPA sights on three of my revolvers I use for competition and the Weigand blade on the fourth. The Weigand blade is aluminum and somewhat delicate - I had Weigand install it on my sight assembly. The LPA is all steel and easily owner-installed on any gun D&T'd from the factory.
Top two are the LPA rear sights, bottom has the Weigand blade...
Here's a little better view of the LPA
Adios,
Pizza Bob