I am often conservative about things like this, unless there's a lot of other money you're hoping to have in.
At a 5% rate, with an additional contribution of £100/month, you're going to have about £944,000 after 30 years, or around £47,200/annum without touching the capital.
Or if you can pay £200/month, after 30 years it's just over a million. If you can wait 35 years at £200/month, that's more than £1.3 million.
And that's really without doing anything, other than not touching the money and investing it with a reasonable, conservative index fund. If you get one pegged to the S&P, which is a more aggressive and undiversified strategy, you're looking at roughly a 10% return. In that case, with £100 month going in, you're looking at nearly £3.7 million, which even drawing down 5% (so, moving it to conservative index funds at that point), you're clearing nearly £185k a year. [Fun: at that rate, with 35 years and £200/month, you're looking at £6.27 million.]
None of this is guaranteed, and also the world has to not fall apart. But still: it's what I would do.
In other words, get ye to a decent financial planner and squirrel away a tiny bit more to add to this nest egg every month.