For most situations a business card about 1/4 the length of the neck pocket is sufficient to give a big enough change of angle of the neck to sort out most problems. The average business card is between 0.333mm and 0.5mm thick (and will compress slightly when the neck screws are tightened up). The only bass I have with a bolt-on neck has a pocket 90mm long. My back of an envelope calculations give an angle of between 0.2° and 0.3°. IME there is far more play in the holes going through the body than that so I wouldn't worry about changing the neck angle with a shim.
Bass Direct in Warwick are the only listed Spector USA Series Dealers in the UK.
However distribution for Spector is by Barnes & Mullins so they should be able to let you know which London shops carry stock.
It looks as though the prices have gone up quite a lot since I sold mine a couple of years ago. Having said that a quick scan through the sold listings only brings up a couple of original Nord Leads and one of those sold for more than a Nord Lead 2 which is a better synth with more features, so there is no real pattern to the prices. I still think that you could get one for £250 max if you were prepared to wait for the right deal to come along.
What you need to go is go to an old-school music shop (the sort of establishment that is termed "Mom & Pop" in the US) and ask if they have one in the box of random guitar bits. If they have it will probably be a bargain as well.
I used to run a true bi-amp set up when it was part of my multi effects (Peavey Bassfex). It allowed me to put all the effects that suck the presence out of the bottom end on the just on the high part of the signal. This fed into a 350 + 350 watt power amp with the upper frequencies going to a home made 2 x 8 cab (based on my favourite guitar combo) and the low frequencies to a standard 1 x 15 bass cab.
I did however mean that in order to get proper FoH representation of my sound I needed to have two DIs on the bass rig.
Basses (or any other instrument for that matter) only sound good on a recording in the context of the arrangement and production of the music. Take a listen to any of the many isolated bass tracks available on YouTube if you want to hear how different (and generally not very good) the bass sounds once you strip away all the other instrumentation.
For I beginner I couldn't recommend a synth that doesn't have a dedicated control for at least 90% of the parameters. Menu access if if you have a page with 5 or 6 parameters at a time gets very tedious very quickly, and will get in the way of seeing properly how the various parameters affect the sound.
Analog or digital doesn't really matter. It's all about whether you like the sound or not. However beware of big impressive sounds when trying out synths in the shop. You quickly find that you have to thin them down in a band situation to allow sonic space for the other musicians.
I'll recommend my personal favourite the Nord Lead. A second hand MK1 version should around the price you want to pay.
I've never been able to get into REM's music. In the late 80s/early 90s my friends all loved them, but I could never hear what all the fuss was about. I'm sure if I heard everything they have ever recorded I could find a handful of their songs that I could stand listening to again, but that's it.
A bit of a mess with the routing for the larger pickup, the bodged second string retainer, and from what I can see the defretting job doesn't look the best (there are no clear photos of the actual fingerboard which is always a warning sign IMO).
But one of them isn't actually playing her bass most of the time.
All those musicians on stage and they still have to have the synth part (which is the main non-vocal tune IMO) on the backing track!
I haven't actually used the HeadRush, but just looking at the photos and information available on the website the onboard user-interface and the interconnectivity options of the Helix appear to be far superior.
Shiina Ringo did a tour and DVD album with the band Hatsuiku Status that featured 3 bass guitarists.
And of course there's Freebass with Peter Hook, Manni and Andy Rourke.
I tried a very expensive Dingwall in the Bass Gallery several years ago. The fanned frets took about 10 seconds to get used to. However I couldn't get any sounds out of it that I would have found useful. Thin and a too "polite" for me.