The wiring regs are not that specific, and can't cover every eventuality. They contain details that competent persons can use to make an installation safe, such as what size and type of cable to use, where you can and can't run & terminate the cables, earthing requirements, overload protection, isolation and the like. But you don't mention any of this in your post.
1. What sort of water heater is connected to the other channel? (gas boiler? immersion heater? other?) and what size of fuse is fitted in the spur? The heaters should have a 3A fuse protecting them and their cables, however, if the spur is also feeding the water heater the total load may well exceed 3A, and you can't just pop in a 13A as then the protection will be insufficient for the 150W rails.
2. The heaters will require a 230V supply. (Assuming they are designed for use in the UK) Why would you want to connect them in "series"? they would each only receive 115v then. They should be connected in "parallel" across the 230V
3. Where will the rails be installed? if it's in a bathroom, building regulations require DIYers to advise their LABC before starting work. They will check the design of the installation and test it afterwards. (For a fee of course)
If you post a little more detail, someone may be able to give a more specific answer.
