Let's offroad / mountain bike / mtb / ride dirt

I was afraid of that.
The reason I lumped the 456 in there is to save cash for a wheelset. Something I know makes some difference.
Bearing in mind my skillset generally means 'point the bike there and hold on'. I don't know if I'd recognize the difference much in terms of speed improvement. Saying that however, I did get to ride an unknown carbon hardtail which was utter bazonks to throw down.

I'll dig more into the Scion. £250 for an 853 would be rude to refuse.

The 456 might feel a bit slack & heavy after the dale. The Soul is a lovely frame, but quite expensive.

It's worth having a look at these two as well:

Pipedream Sirius & Scion or Dialled Bikes Prince Albert

Both more nimble in terms of their geometry and with nicer tubing than the 456 and cheaper than the Soul.

@BareNecessities

Not to be a chuffnubbin or anything, but the Dialled PA is 'Reynolds 725' steel.
The On One 456 is 'DN6' Steel (named after On-One's postcode)

No matter what the name is, both of these are exactly the same, 4130 (this is the industry standard AISI designation) double butted steel.

http://www.finishing.com/323/64.shtml

http://www.on-one.co.uk/i/q/FROO456E2/on-one-456-evo2-frame

http://reynoldstechnology.biz/assets/pdf/rtl_steel_alloys_extract.pdf

Interestingly (ok, not very) scroll down to the bottom of the Reynolds info and they have a stiffness chart, all of the steels are shown to be stiffer than the 6061 (ok, so there's 7005 as well but I didn't notice any info on that) which is the regular Alu used in bike frames. And Aluminium frames get slated for being 'stiff' and steel is supposed to be 'compliant'.

It's all a load of old trousers, but @Chak I found my On-One seemed to feel larger than it's 16" frame size should have.

Good Luck