I'm looking at doing the 500 next year (or more likely 2021). Is it worth tagging Detroit onto the end of the trip, considering I'm coming over from the UK.
If you can afford it, I'd say so, yeah. I've both worked and spectated it. It's not your 'normal' race experience but you'll certainly have a lot of fun!
The Detroit/Belle Isle weekend packs two street-course Indycar races and an IMSA sprint race in, so you definitely get your money's worth and plenty
of on-track action from all kinds of cars. Stay in Taylor, Michigan for the weekend if you want a cheaper hotel; it's like 20-30 minutes from the track, which you'll need to catch a bus over the river to get to. Also be sure to get breakfast at Tim Horton's at least once while you're there. Digging into a cup of Timbits while you watch the sun rise over Motor City is really the highlight of the whole experience.
Be sure to explore the infield if you go because amusing weirdness will inevitably abound.
If you're looking for random-ass stuff to do while you're there that you didn't know belonged on your bucket list, the Mariner's Church
of nautical disaster fame
is mere minutes from the track.
So yeah, I'd say if the opportunity presents itself, do both. You'll probably remember it the rest of your life if you do.