If, like me, you are lazy (and not confident in managing and balancing your own investments), then Vanguard Lifestrategy through Fidelity (fidelity.co.uk) is a good choice. They have a balance of UK and international indexes, and automatically rebalance to keep your allocation roughly the same. Here's an overview of the Lifestrategy funds for the UK:
https://www.vanguard.co.uk/uk/portal/investments/all-products?assetType=BALANCEDThey are divided between equity (stocks) and bonds. Equity has bigger risks and bigger rewards. Bonds are seen as safer, but with historically lower rewards. People tend to adjust the allocation as they get closer to FI – so you might start off with 80% equity/20% bonds, and move through 60/40, 40/60, 20/80 as you get older. I have 100% equity at the moment because I have relatively high risk tolerance and I'm a long way out from retirement.
So, choose a fund, open an ISA with Fidelity, set up a direct debit – that's how I did it. Bear in mind that if you chuck all your savings and monthly contributions at it, you'll exceed the ISA limit of £15,240, so if you want to get heavily into Vanguard and won't need the cash then you could put in £3,240 up front and £1,000 a month thereafter (assuming you haven't paid into an ISA in the first few weeks of this new tax year).
With the extra £200 a month you could put it in a SIPP or just keep it in cash at 4%. In fact if you're a higher rate taxpayer SIPPs become more attractive because they're essentially pre-tax contributions. I'm a basic rate taxpayer, so I haven't looked into it too closely.
Caveat lector: I am by no means an expert – this is just my experience and comes with no warranty, express or implied. Use anything here at your own risk.
