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There are a few ultra-budget frames on CRC for ~£200
http://www.chainreactioncycles.com/vitus-nucleus-vrs-29-hardtail-disc-frame-2018/rp-prod161848
http://www.chainreactioncycles.com/octane-one-prone-27-5-frame-2018/rp-prod155476
http://www.chainreactioncycles.com/vitus-nucleus-vrx-27-5-hardtail-disc-frame-2018/rp-prod161849
(there are more, I just posted a selection)
Could get you up and running for a few months/a year then you can swap the frame for something better down the line when you know what you want. The only problem is they look to be mostly post-mount but MTB brakes are cheap and there are flat to post adaptors (I think?) if you switch to a flat-mount frame. Aside from that everything looks pretty standard so I would think you wouldn't have much trouble swapping nearly every part over to a new frame*. Just pick the wheelsize and fork travel that you want to stick with now.
* could get some wheels with replaceable end caps and you'd be fine for 135mm QR and 12 x 142mm. Maybe go for 27.5" wheelsize, then you can use them with your Arkose with narrower tyres if you feel like it.
Is this a stupid idea? Someone tell me.
You shoulda bought that £50 29er frame you posted the other day