The 'for Britain' grouping are reformers. They don't want 'out' they want 'associate membership' that hasn't yet been publicly defined and can't be implemented in the time available before a referendum.
Matthew Elliott, leader of the 'for Britain' groupings, saw the 'Yes2AV' campaign fail because it was led by the Electoral Reform Society (ERS) who were *against* AV, they wanted (and still want - the now obsolete) STV.
Matthew Elliott now wants to recreate this in the EUout campaign - by leading it to fail, by leading it as badly and half heartedly as ERS led Yes2AV.
Just before the referendum, if the in campaign have not already clearly won, then the EU will offer 'associate membership' they will promise it as a 'vow' (as per the Scots independence 'vow' - the promise of Devo Max if scots voted 'in') and the reformers led by Elliott and 'for Britain' will switch from EUout to EUin - this is what they wanted all along (see http://www.standard.co.uk/comment/comment/isabel-oakeshott-eurosceptics-need-to-wake-up-if-they-want-us-to-leave-the-eu-10289339.html)
The reason purdah has uniquely been abandoned for this referendum is precisely so this last minute Damascene moment can be stage managed - in the last week or so this offer will be made and hugely publicised - and the UK public will be expected to vote 'in' in a promise (cast-iron no doubt). With (but this time) only UKIP still speaking up for out!