Find a cheap discount brokerage, for example iWeb Share Dealing (there is a comparison list of the UK brokerages somewhere). Open an ISA. Pay in your 600 quid a month. Figure out an asset allocation (eg, 10% VUKE, 20% VEUR, 20% VFEM... etc). Buy in one transaction one ETF every investment (unfortunately trading costs in the UK are relatively high; ideally you want to buy 1k GBP worth at a time, as the trade will cost you 5 pounds).
Then sit back, and watch your money grow. Or, even better, don't watch, just rebalance to you asset allocation as you invest.
The ISA is awesome but "use it or lose it" so you want to get as much in each year as you can. The max if you can.