I don't know if the comparator OAL is about right, but the 155 Scenar is a VERY long bullet for its weight and is a near VLD type, albeit with a less aggressive secant ogive profile than a 'true VLD'. (10.7 cals radius compared to ~14R for the JLK and Berger 155gn examples). Bullets with secant ogives tend to produce a rather higher comparator reading on hitting the lands than an equivalent tangent ogive type.)
I'm assuming your TR rifle was built pre the current GB NRA contract with RUAG so the chamber would be throated to suit the old RG 155gn sniper round which doesn't need much freebore, pretty well as the original 155gn Sierra MK (#2155). As such, you'll end up with the Scenar seated pretty deep. Nevertheless, it was a very popular handloaded bullet here in the UK for many years, that and a hefty charge of Viht N140 (at least 45.0gn and more often up to 46gn and a bit) was a staple combination - but don't start up there, work up to it from maybe 43 or 43.5gn.
A friend and I got a reamer from PT&G based on the Scenar seated to the ideal position with the bottom of the shank at the case shoulder-neck junction in the early days of FTR, pre the appearance of the 155.5gn Berger Fullbore. Unfortunately, I don't have a note of the COALs, either comparator based or true overall, but was certainly on the long side of 2.9 inches.
When the 185gn Berger LR BT appeared, the freebore that suited the Scenar turned out to be exactly right for this bullet too, and in due course when I rebarreled to a 1-10 twist tube still with the same chamber, it was just usable with the 210gn Berger LR BT and 208gn Hornady A-Max, although they were seated deeper than was ideal. So, the chamber that suits the Scenar is pretty long-throated, much more so than the Bisley 150 and Palma 95 chambers.
Having said that, it never stopped our TR guys with their Bisley 150 / Palma 95 chambers handloading this bullet and getting it to shoot well.
VarGet will work, but H4895 and IMR-8208 XBR may work better and give higher MVs. As noted Viht N140 works very well, also N540 for those looking for more velocity. The amount of 'jump' that's ideal is debatable. I'd start with at least 0.010", would prefer 0.015" or even 0.020". Some people get better results putting it into the lands a few thou' no doubt due to its quasi VLD form.
As you had the barrel put on at Bisley, it'll likely be a slightly 'tight' example compared to the SAAMI norm, the GB TR community (like many Canadians and some US Palma / Fullbore shooters) still prefer a bore dia. around 0.298" and groove diameter of 0.3075" or similar. This and the short throat raises pressures, so be wary of some loads quoted by FTR shooters with their standard SAAMI .300/.308 barrels, even more so by some people on this forum using stock .308 Win Remington 700 PSS rifles who'll tell you to use frighteningly heavy charges of VarGet.
