In my post this morning I received a 'National Notification of New �5 Coin' 'to celebrate the 80th birthday of Queen Elizabeth II'. This is the offer: "If you reserve your coin today it can be yours for just its face value - �5 for �5. POSTFREE."
All I can think is they hang on to all the fivers for the interim but assuming you get your coin in April they'll only earn about 15p and that won't pay the postage. Not necessarily dodgy but worth more investigation.