Thanks for all of the replies. I've sent (far too much of) the day researching these beauties some more and trying to decide which path I should go down. I've put my thought process below, for anybody to point out any oversights (As can be seen by the time I've spent considering this, I'm really open in trying to find the best for me).
First, I considered UK distributors (Price includes shipping):
New IT SheevaPlug (Dev)
£89
(£95, £105 & £149 available)
Dimensions (L*W*D): 4.33" * 2.74" * 1.91"
And then US distributors (Prices include shipping):
Ionics (Dev)
$129.00
(£78.15)
Dimensions (L*W*D): 3.03" * 1.97" * 0.28"
TonidoPlug (No Dev)
$141.50
(£85.72)
Dimensions (L*W*D): 4" * 2.5" * 2"
GlobalScale (Dev)
$149.66
(£90.66)
Dimensions (L*W*D): 4.33" * 2.74" * 1.91"
QuadPlug (No Dev)
$170.00
(£102.99)
Dimensions (L*W*D): UNKNOWN
CloudPlug (No Dev + SATA)
$218.00
(£132.07)
Dimensions (L*W*D): UNKNOWN
From this list, it appears that only Ionics and TonidoPlug are viable (Others are too expensive compared to the UK distributor).
Additionally, I know that whichever route I go down, I'd like to install Tonido Embedded. Considering the cost of this:
Tonido Embedded License
$29.99
(£18.17)
Considering the costs with this included (TonidoPlug comes with this and therefore the price does not change):
New IT SheevaPlug (Dev) + Tonido Embedded License
£107.17
(£113.17, £123.17 and £167.17 available)
Ionics (Dev) + Tonido Embedded License
£96.32
TonidoPlug (No Dev)
£85.72
Adding on 15% VAT (On the actual item price, not delivery) (New IT are a UK distributor and therefore VAT is already included):
New IT SheevaPlug (Dev) + Tonido Embedded License
£107.17
(£113.17, £123.17 and £167.17 available)
Ionics (Dev) + Tonido Embedded License
£105.31
TonidoPlug (No Dev)
£94.98
Considering shipping time:
New IT SheevaPlug (Dev)
1 day
Ionics (Dev)
28 days
TonidoPlug (No Dev)
3 - 4 days
Leading to overall pro's and con's of each:
New IT SheevaPlug (Dev)
Pro's
+ Shipping time
+ No customs/import tax
+ Convenient returns (30 days)
+ Development model
Con's
- Most expensive
Ionics (Dev)
Pro's
+ Small
+ Development model
Con's
- Customs/import tax
- Permanent plug
- Shipping time
- Protruding male JTAG
- Inconvient returns
TonidoPlug (No Dev)
Pro's
+ Shipping time (Considering US distributor)
+ Tonido Embedded pre-installed
+ 5-year SSL license (Though this confuses me; Doesn't seem to use trusted identification. See here:
http://www.tonido.com/support/InstallTonidoLicense)
Con's
- Customs/import tax
- No development model
- Inconvient returns
With these considerations the Ionics can be discounted from any decision.
I'm now left having to decide between going with New IT or Tonido. Tonido seems cheaper and includes Tonido Embedded pre-installed however New IT model has the additional development IO's. I am not sure how much weight is held by the fact the TonidoPlug comes with a SSL license. Difficult decision!
And
yes I think I did consider this in a little
too much detail!

Cheers!