My plan in the future is to possibly add IP cameras for CCTV so I could do with at least 4 x POE ports. or in future I could buy a 4 port POE switch and then plug that into the router?
Also is the ER-X fast enough for a 300MB Virgin Media connection?
Plenty fast enough https://www.stevejenkins.com/blog/2017/02/edgerouter-x-vs-edgerouter-lite-google-fiber-speed-tests/
Yes you could add a POE switch at a later date no problem.