Is a Salsa Backborrow frameset obtainable in the UK ?

[RufusChucklebutty]

Is there an alternative cargo bike frame available?
Could a steel frame be converted to mid lengh cargo ?

 

[All uphill]

Is there an alternative cargo bike frame available?
Could a steel frame be converted to mid lengh cargo ?
View attachment 681225

No idea, but I want one!

 

[glasgowcyclist]

Is there an alternative cargo bike frame available?
Could a steel frame be converted to mid lengh cargo ?
View attachment 681225

By ’eck, that’s a tiny chainring!

 

[Cycleops]

....and what a long chain you've got grandma.

 

[berlinonaut]

Is there an alternative cargo bike frame available?

It probably depends a bit from what you are aiming at. These kind of bikes are called "long tail" and while they are less common than other cargo bikes there are a bunch of them on the market. However, the salsa looks like a 29" MTB in form of a long tail and while other bikes like that may exist I am not aware of any - which does not mean much.

What comes relatively close is the Surly BigDummy: https://surlybikes.com/bikes/big_dummy and it's electric siblings BigEasy and SkidLoader: https://surlybikes.com/bikes/haulin

Then there is the Yuba Mundo: https://www.yubaeurope.com/en/bikes-add-ons/mundo/mundo-lux

What you could also look for is a used Kona Ute - unfortunately no longer available new but relatively close in terms of design and typically not very expensive:
https://konaworld.com/archive/2016/ute.cfm

There is also a longtail available from Decathlon which seems to be very good based on owners reports:
https://www.decathlon.co.uk/p/electric-cargo-bike-rear-loading-longtail-r500/_/R-p-329177?mc=8605830
In the UK it seems to have a hefty price tag (in Germany it ist 2900€ and used to be 200€ cheaper). Over here the frame is also available separately as a spare part for 700€: https://www.decathlon.de/p/rahmen-f...bike-r500e-longtail-hellgraublau/_/R-p-336188
However, the (morotized) rear wheel is 20" and the frame has a dedicated place for a battery - but surely you could build it up as a non-motorized bike if you like.

Then there's the Bike Friday Haul a Day, 20" front and rear, frame set should be available:
https://bikefriday.com/product/bike-friday-haul-a-day-cargo-bike
https://bikefriday.com/service/haul-a-day-frame-specifications
https://www.wired.com/review/bike-friday-haul-a-day/

There are many more, but typically not avail. as frame set but only as complete bikes and typically with an electric motor.

Could a steel frame be converted to mid lengh cargo ?

Until a couple of years ago that was what xtracycle offered: An extension to mount to the frame to convert it to a longtail cargo bike.
https://www.happybikes.jp/product/1740

Over the years, there were various different kits - this is what typical examples looks like:

However, xtracycle have switched to building complete bikes some time ago: https://xtracycle.com/

But if you are lucky you still may be able to find one of their conversion kits new or used. The xtracycle desgins were published as open source at some point in time to stimulate the cargo bike market but seem to be no longer available: https://en.wikipedia.org/wiki/Xtracycle

 

[berlinonaut]

Only just realized that the Salsa Backborrow is a fat bike: https://www.salsacycles.com/bikes/fat-tire/blackborow
A hint to this fact or a link would have been helpful and would have avoided a long post with probably not so helpful information for you...
However: Salsa do have an UK-Distributor according to their webpage: https://www.lyon.co.uk/
He should be able to tell you how to obtain a Backborrow.

 

[RufusChucklebutty]

They are all way out of sensible price range , I'm going to have to build a frame or nothing.

 

[berlinonaut]

They are all way out of sensible price range , I'm going to have to build a frame or nothing.

You did not mention the price range to be a criteria until now. There is the Yuba Kombi which, as a complete bike costs 1399€:
https://www.yubaeurope.com/en/bikes-add-ons/kombi/kombi
If this is still too expensive you don't have many options other than build one yourself or to watch out for an xtracycle conversion kit or a used Kona Ute. If this is still "way out of a sensible price range" possibly the price range is somewhat unrealistic and adding a trailer to your existing bike is the more realistic solution.