Probably increased IT investment would be the default option,...

Probably increased IT investment would be the default option, and if it's too tough/slow to develop in-house, then acquire it. They haven't shown a pattern of acquisitions, so I'd assume they'll develop most of what they need.

Customer requests -

International trading - can be done in-house in under a year, just requiring a good partner.
Faster transfers - can't imagine that an acquisition would solve this, but I see IG can accept debit card for free. Even a $1-2 surcharge or something on debit card transfers would be acceptable for lots of users, if it was instant. For people who want to save money, they can keep doing bank transfer as usual.
Live data - just a matter of cost. Acquisitions won't help.
Tax reporting - maybe they can get away with in-house development for a good-enough level of reporting. I imagine something like ShareSight would be complicated to maintain anyway.
Better research - they're already working on it, just requiring a good quality and good value partner.
Better app - already working on it in-house.

I come back to the same conclusion, that if they're accumulating money above the $5m balance, it might mainly make sense to hire more devs to improve the website/app faster, and more CS to keep up with demand/growth.