Welcome to GoY Budding.
keeping the flower is a personal preference. if you like them, keep it and when it finishes flowering cut it off as low down as you can. If you don't like it remove it from as low down as you can.
Either course of action wont affect the plant. They tend to flower mid summer in the UK . You've not clicked a location on your profile so that may affect when it flowers.

If its a Cordyline (and not a Phormium, the two are often confused) it does not need any pruning at all. I concur with the advice already given in regard to flowering....

Thanks for your comments. I live in Lincolnshire and it’s flowering now, but I don’t know how long it takes before the seeds become a problem. I don’t want lots of seedlings and neither do the neighbours..It’s about 7 feet tall, so already need a pair of ladders!
I inherited the plant when I moved here, and it hasn’t flowered before.
Thanks.

as soon as the flowers fade cut it off and this will stop seed forming. if you don't like the flower cut it off when you get the time/inclination.

I've never known the seeds to germinate - lots of Cordylines here in London are very tall indeed and the flowers are impossible to reach without very long ladders, so they just get left to seed and eventually the dead cluster of flowers gets blown down by the wind. Certainly not an issue with lots of seedlings,never seen any at all, the biggest problem is the dead clump can be quite ugly after a few months if anyone notices them amongst the leaves.