Growbags? Shove in some tomatoes? Who is going to nick. If they get vandalised / removed not much lost.
I have chillis I've over wintered. They are great. Wouldn't not have them. Mine will go back outside when its warmer. Just go for a shorter variety (I've a couple which are a lot taller than I'd anticipated so my spare room looks a bit like a jungle atm).
I'd pick a windowsill size tomato plant as a must too.
See something like this. There are a few varieties about:
www.suttons.co.uk/vegetable-seeds/seeds-for-kids/tomato-seeds-table-top-tomato-sweet-n-neat-red_mh-44871
As someone has mentioned, in case anyone else wants to try it, I've done tomatoes in hanging baskets before. They are brilliant. Standard hanging basket with 3 cherry lizzano plants in. Plant now, you'll have tomatoes from June to October. Really nice flavour. They probably a touch too big for the window sill though.
www.premierseedsdirect.com/product/tomato-cherry-lizzano-f1/
Other things I do for a window sill:
Rocket, coriander, parsley, spring onions, basil as my basics.
I'd consider some lettuce like this, which can be cut young:
www.premierseedsdirect.com/product/lettuce-gourmet-looseleaf-cutting-mix/
I grow a fair amount in pots, and its surprising how much you can do. Anything that will grow to required size which fits into a 20cm pot will work for a windowsill. Strawberries would work.
Ikea do these little citrus plants which I do fancy trying at some point:
www.ikea.com/gb/en/p/citrus-potted-plant-calamondin-30540361/
www.ikea.com/gb/en/p/citrus-potted-plant-lemon-10540362/
Again they'd work on a window sill.
I go for 30cm pots for patio stuff (blackberries, blueberries, raspberries, aubergines, courgettes, squash, cucumber all work on patio in the right pots. Blackberries and raspberries go well in hanging baskets).