Above link does seem to explain a lot and also why you can not invest more i.e. too much money would come in to the business. Everyone wants to invest more (including me 
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Loads of ways to grow the £1,5k you want to invest now and hold it until the 2018 round which @tristan talks about yet which someone else also mentions is fake news? 
e.g. a low risk option; shuffle your personal finances > with Faster payments and the Monzo current accounts sending money back and forth to a nationwide flexdirect account will allow you to get 5% over any amount up to £2,500 (have to pay in at 1k each month but shuffling money means you can retain the rate and keep savings up to that amount in there) Or you can think of an IFISA (e.g. Crowdstacker), which has good returns, lending out your cash via Zopa or via other platforms in the UK (or NL, DE etc.). Decent returns, medium to high risk and you’ll have more Monzo shares in next year’s round 