Anyways, by distilling the info from several sites, I sussed it out. Here it is:
- Open phone app
 - Preferences & Accounts
 - Scroll down to Network
 - Switch Manual Settings on
 - Tap Edit Network Settings
 - Tap MMS APN
 - Set MMS APN to goto.virginmobile.uk
 - Set USERNAME to completely blank
 - Set PASSWORD to completely blank
 - Under MMS PROPERTIES
 - Set MMSC to http://mms.virginmobile.co.uk:8002
 - Set MMS PROXY to 193.30.166.2:8080
 - Leave MAX SIZE at 300
 - Tap Change Settings
 
And that should be it!