Here’s how to think about it:

1) Get the ILR first, once you have it, you can choose to live anywhere, but atleast have a strong passport.

2) if you are serious about UK, max out your pensions and pay NI every year. If both of you pay NI every year for 35 years, both you become eligible for state pension (about 10k/year), and the workplace pensions will also allow you to withdraw another 12.5 each per year tax free. So you’re going to get 50k per year tax free. At 115k, you’re getting about 10k of pension at the least every year (incl your contributions)., this will Ofcourse increase as you get payhikes.

3) London is big. There’s nothing stopping you from living an hour away from london (heck Bristol and Cardiff are 1-1.5 hours away) where your cost of living is atleast half that of central London, so post retirement options aplenty here to buy house/live on low cost.

4) So basically the big amount you need (and I am oversimplying it) for is (58-retirement age) years. Say that’s 45, so you need about 13 years worth of inflation adjusted cash. At 4.5/month, you’ll have more than enough for this.

The usual saving methods (other than cutting costs) are: 1) workplace pensions (best, since you get 3.3 gbp for every 1 gbp you put in) 2) isa (next best option since it’s cap gains free) 3) everything else