I emailed Worcester's Technical section to ask about this - this is their reply:
"The boiler would essentially work for direct hot water (DHW) with the flow and return isolated and with pressure still in the boiler, however there is no integral bypass within the boiler meaning the chances are...
Thanks for your replies.
I'm a bit worried now as I have already done as Agile suggested - turned off the CH flow and return valves, although I haven't drained down the system yet.
The hot water has been used continuously since, for washing up, showers etc., and I've not heard any unusual...
I am about to replace several radiators so will be draining down the system for about a week.
Will I still be able to use the hot water system? If so, do I need to do anything apart from turning the CH control to off?
My boiler is a Worcester Bosch Greenstar HE combi, model ZWB 7-27 HE...