Thanks for the responses and suggestions people.

@Jur
Yea I saw those, it's an SRF5(W) I believe? I'm just not sure if it will fit onto my brompton as the mounting bit for the pulley housing on my rear triangle had been cut off and no longer available unfortunately. see photos attached.

@Dynaryder
Thanks for the ideas, I think the P-clips would work fine.
Sure I'll take some pics of the stem for you when I get spare time this thursday.

@blakcloud
Thanks for the tips, you're correct. It is a lot of work but I don't mind getting my hands dirty if I can get close to the gears and range that I need, plus I will understand a bit more about the build and maintenance of a brompton. I could potentially sell it and buy a new 6 speed but what concerns me with the BWR and 44t combo is the fact that it might not be low enough for me so I will have to end up customising it again anyway and 39t ring from Tiller Cycles is the lowest I've seen so far.

I had another look at the 8 speed kit from kinetics and apparently it does not require any modification of the frame. Looking at the picture, it also doesn't seem to use the mount for the pulley housing so this might be the way to go.

Otherwise as you suggested I might sell it, get a used 3 speed and add mountain drive but that might end up costing more than the 8 speed kit above. I'll do a bit more research.

@fietsbob
I think the gearing on your bike is pretty much the gearing that I'm after really. The only issue is mounting the BSR pulley onto my brompton decapitated triangle.
I'll see if it's possible to go with a 25t with the 8 speed. I know someone had done it with a 23t though. Do you know if the chainring can go any lower than 33t?

@mulleady
Yes you're right, it's important to respect the owner as they each have their own personalities, quirks and priorities.
I bought the bike as is because it had been maintained pretty well and it was in a great condition, a lot of the used bromptons I saw were rather... for the lack of a better word, rusty & crusty. I also liked the added adjustability of the modded stem which gives it an inbetween an S and an M bar height. But honestly if it wasn't for the missing "limbs" on the bike's frame, I would've just bought a 3 to 6 spd wheel upgrade and I'd have myself an almost brand new 6 spd brompton at 3/4 of the price. I just didn't realise it at the time and unlike you, I am not lucky enough to live anywhere close to the Brompton motherland.
The BWR hub you suggested unfortunately would require the use of the mount for the pulley housing on the rear triangle which I don't have. Unless you have an idea on how to affix the pulley housing onto the triangle securely by any chance?