This is in the Terms and Conditions at
http://www.dollar.co.uk:
Cars are rented with a full tank of fuel (customers will be charged at pick up unless they have purchased the Platinum Package) and should be returned empty. No refund for unused fuel.
I never understood why they did this, but as far as I've ever understood the overseas prepaid voucher program, no matter how you slice it or dice it, prepaid gas is mandatory. The only difference is that with the regular package, you pay for the tank at the time of rental, and if you opt for the Platinum Package (which has a higher daily rate), the prepaid gas cost is included in your voucher.
Based on my reading of the prepaid voucher program, the agent is indeed correct when saying "You MUST pay for a full tank now." That is built in to the terms of the voucher.
FWIW, the voucher program is a PHENOMENAL deal for overseas renters. Assuming a base rental rate of $30 per day--on the low side these days--and a coverage package cost of $40 per day ($25 for LDW and $15 for SLI), with an average 30% in taxes, if you were booking directly with a rental agency and buying the coverage separately, you'd be looking at an approximate cost of $80 per day.
That same vehicle including all of those coverages goes for less than $50 per day through the prepaid voucher program.
One location I'm familiar with actually stopped participating in the overseas tour vouchers because they basically ended up renting cars for free, the voucher only paying the cost of the coverage package. That, combined with the damage they had to eat from some of these rentals, meant that they actually lost money on the program. They're not in business to lose money (though it's almost unavoidable in the rental industry)!