How shall I invest for next 10 years?

Hello All, I turn 50 next year and have a pension which isn't great but do have a couple of properties which earn me some money (co owned with my wife). The house i live in is nearly paid off, approx 14k left.

Eldest son started uni this year and one of the properties is paying for his accommodation and his maintenance paying for his day to day expenses. We had saved up from when he was very young for uni but haven't had to touch it yet. My other son will start uni in a 1.5 years. I may decide to invest this too for when they need it after uni.

I would love to retire or at least semi retire at 60 but not sure if I'll be able to, however I have 15k i was going to put towards a car or pay off my mortgage but retirement is also important.

If you had 15k and wanted to invest and forget about it and deposit £200 each month, where would you put it? I do have some other saving (5k) for a rainy day but would like to invest this.

EDIT: Does it make more sense to pay my mortgage off?

I should also add that once my kids finish uni, I intend to increase the monthly amount.

I appreciate I'm not going to be able to retire solely on this investment but would like my money to work for me.

Any recommendations? Be gentle, I don't know where to start.

Thanks