First, repeat advice to sell the car and get something cheaper. But I don't know anything about the UK car market, so for all I know that cost may be reasonable.

So second: the math says to invest more. Period. Your interest rates are very low, especially when viewed historically. It is therefore very likely that you will do better putting that money in the market.

In addition, when you invest, the first few years make a huge difference in your end results. The rule of 72 says that, if you get a 7% return, your money will double every decade. So if you have $50K invested now, in 30 years that will be $400K (three doublings).* OTOH, if you start a decade later, in the same 30 years your money will be worth only $200K -- **half** as much. Think about that: same actual amount of money, put in exactly the same investments, but you end up with only half as much, just because you started later. So you want to get as much invested as early as you can, so your money has as much time to grow as possible.

The complicating factor here is the UK mortgage system, which I understand offers only adjustable-rate mortgages, not fixed ones. But my advice remains the same, because if rates skyrocket like they did in the late '70s, you can always sell your investments and pay down the mortgage.**

*Ignoring tax implications here for simplicity's sake.

**Yes, it's a little bit of a risk (because skyrocketing inflation tends to depress market performance, so you might be selling at a low). But it is fairly unlikely that that would happen quickly enough that you'd get totally hammered. So I'd go after the option that is much likely to happen, i.e., market continues to outpace mortgage rates over the longer term.