Just did a UK search... I can buy it on-line and I guess if I had a good butcher I could ask for it. Here is how one on-line supplier describes it:
And again:
https://www.turnerandgeorge.co.uk/tri-tip.htmlAn old fashioned cut that was usually wasted, the tritip is now valued for it's great marbling and flavour. Versatile to cook, you can BBQ as steaks, or braise slowly
And again:
Inspired by American traditions of butchery, the Tri Tip is taken from the sirloin, and though smaller than other sirloin cuts, promises the characteristic marbling and natural succulence of our grass fed beef. Our Tri Tips are exclusively dry age matured for the most tender finish possible, and as such are ideal for roasting the British way, served with all the trimmings.