Bebs,
When you are talking BT you are talking about BT Vision.
BT Vision is a digital TV service which offers digital TV (using Freeview via a TV aerial), a digital video recorder (called a V-Box), and supports TV and video content that's accessed on-demand over a BT Broadband connection.
Many see Television-over-Broadband (known as IPTV), or Video-on-Demand as the future of television. BT Vision's offering combines Freeview, a hard-disk recorder (for recording and live pause), plus access to on-demand content.
So, you will need a good standard aerial for BT Vision which provides the Freeview channels which you can get with any Freeview STB.
What it provides extra for a charge is "On-Demand" programmes and films. Have a look HERE for full details.
I would suggest that as you have a Sky box which currently works with your Beo4 via the STB controller you keep with it provided you are allowed to use a Sky dish in your flat