I'm a BE customer. BE have supplied a number of modem/routers over the
years - all from the same company, variously known as Speedtouch/Thomson/
Technicolor.
Some of these models have locked firmware - in the sense that you can
only install firmware provided by BE, and specifically *not* in the sense
that you can only use the firmware on a BE connection.
Other ISPs do this as well so you have to use firmware which they
provide, but there's nothing to stop you from using their firmware with
any other ISP. Other ISP use generic firmware, and you can't install an
ISP's locked firmware on such a modem/router.
I suggest the solution is to ask Zen if they have a template for the
model concerned. If so, install that template and use it in the wizard
to configure the device for Zen. If there's no template, it's worth
looking at
http://forums.modem-help.co.uk/viewtopic.php?t=9301
If you're brave and have a JTAG cable, it's possible to unlock the device
so that you can use generic firmware on it - Google for "firmware unlock
TG585v7" (or whichever model it is). But generic firmware might not be
the right answer for you - e.g. do Zen provide firmware, and if so is it
locked as well (so you might want to unlock from BE and then lock to
Zen) ?