McCreadies at Rugby, or used to be, then called Niagara and can't remember what they are called now.
Phone number is in workshop.

Just done a search, now called Acenta Steel

http://www.acentasteel.com/distribut...cts/index.html

Alloy steels in second column down RH side. Just noticed that they now only do EN19 in 80mm diameter upwards, so if i had to buy some I'd go for EN16M or EN24, both would be suitable.
In fact for a home shop EN8 would be fine and it machines like butter.