There's no specific rule for the order of transmissions so far as I've noticed think the norm in the London TMA is Heading then Level. Any speed instructions would be added at the end.
There is also some Human Factors impact on the order of transmissions. We are not supposed to issue more than 3 executive instructions per transmission and many people start to panic after the second! The above order works reasonably well with this as the speeds flown tend to be pretty standard so demand less active thought to remember for the period required.
Furthermore, in a string of instructions the item most likely to be recalled is the final one, and as a wrong level will more often than not result in a quicker loss of separation, adding it at the end may also have safety benefits
CT