Offering an applicant on the basis of a reference sounds pretty mickey-mouse to me.
UCAS rules are quite clear on this matter:
if you accept an unconditional offer you cannot hold another offer as an insurance
if you accept a conditional offer you can hold another offer as an insurance (in the event you don't meet the conditions - which are normally a minimum number of tariff points from results)
But your 'offer' appears to be conditional only upon their decision about a reference - in other words, it is out of your control.
UCAS information on conditional offers is here.
http://www.ucas.ac.uk...nts/offers/offertypes
http://www.ucas.ac.uk.../replyingtoyouroffers