Your Buffalo router has an Ethernet WAN port, making it what is sometimes called a cable router.
With the current engineer install FTTC or FTTP setup, you could connect the Buffalo router's WAN port to the FTTC modem or FTTP ONT. For most ISPs, you configure the router for PPPoE using the ISP supplied user name and password. TalkTalk and Sky use DHCP rather than PPPoE. So far as I know, TalkTalk uses standard DHCP. Sky needs authentication details passing in the DHCP options in a system sometimes called MER (I'm not sure whether they hand the details out on demand yet, or whether you still have to obtain them from your Sky supplied router).
Before long, self-install FTTC is likely to be here, and this may well become the norm with most consumer ISPs. In this scenario, the engineer does the work at the cabinet, and the ISP supplies the necessary filter(s) and router or modem for the user to connect themselves.
So far as the choice of ISP goes, there's plenty of options. Which option is best for you depends on what you are looking for from your ISP, and what services you wish to purchase along with Internet access.