I love this game a lot! It's a really fun tower defense game that wears a lot of Bloons inspiration on its sleeves, while putting it's own spin on it - I like how the bomb tower isn't just a bomb shooter, but lays delayed bombs, for instance! The currency being pounds is also really funny to me, for some reason.
Honestly the only thing I felt was lacking was an upgrade system, which you've already thought of but couldn't implement. Maybe a boss fight of some kind to end a run, maybe some sort of platonic solid?
Overall, it's a fantastic game, and I'd love to see it expanded if you feel like pursuing it!