What you want to do is point the DID to your SIP URI...
From the looks of it you won't be able to do that with UKnumber.co.uk, but you should be able to point your VoipUser number to
user@sip.justvoip.com ...
VoXalot will be neat to add to the combo especially if you plan on getting more complicated later....but the VoipUser DID should be able to forward to either your JustVoip or VoXalot SIP URI.
You may also be interested in taking a look at this...
http://forum.voxalot.com/voxalot-gen...voip-info.html