UK Procedures
Procedures vary amongst UK airlines, but generally they require cabin crew to remain seated after takeoff until a signal is received from the flight crew allowing them to begin the cabin service. This signal could be a temporary off/on selection of the fasten seat belt signs, or it could be a brief PA such as 'cabin crew released'.
Now, the fasten seat belts signs may still be illuminated at this point, possibly because the pilots expect the aircraft to encounter turbulence or possibly because company policy dictates that the signs must always be 'on' whenever the aircraft is below a specific altitude or flight level.
At other times, such as when the aircraft is above the specified altitude or flight level and turbulence is encountered or anticipated, after the fasten seat belt signs have been switched 'on' the pilots will probably not instruct the cabin crew to secure the trolleys and strap in unless they (the pilots) consider that the turbulence is likely to pose a particular threat to cabin crew safety. Then of course, they will instruct the cabin crew to do just that.
There is no '10,000' ft rule in the UK relating to cabin crew movement. If there were, on some of the shorter sectors that are so great a feature of European commercial flight operations, there would be scarcely any service at all!
Hope this helps.