>Surely the prizes are based on the amount of money raised by ticket sales .
Yes. According to Wikipedia:
Of all money spent on National Lottery games, 50% goes to the prize fund, 28% to 'good causes' as set out by Parliament (though some of this is considered by some to be a stealth tax[2] levied to support the Big Lottery Fund, a fund constituted to support public spending[3]), 12% to the UK Government as duty, 5% to retailers as commission, and a total of 5% to operator Camelot, with 4.5% to cover operating costs and 0.5% as profit.[4] Lottery tickets and scratch cards may be bought only by people of at least 16 years of age.
So if the receipts double the prize fund should double, although that doesn't mean each prize will double.
My guess is receipts (and therefore the prize fund) will increase by no more than 20%