I read that the restriction had been changed. This is no longer a "total amount for the fund" but rather an application date in May.
The deadline to apply is 15 May, with the Government expecting a total of £15 billion to be deposited by that date. It had initially not put a time-frame but had limited their availability to £10 billion in total deposits. That would have been enough for 500,000 people if all paid in the maximum contribution, but by early February, £7.5 billion had already been deposited by 610,000 people, prompting the Government to extend the scheme so more people could invest.
From Moneysavingexpert.com