Is there a maximum length for a spur cable?

Dennis, I am not sure that this is a "better way". There is no reason in the regs why I cannot put a spur to my shed, whether it feeds a FSU and a light or not. It should be RCD protected, as I assume we are talking about the 18th edition. It is no different to using an extension cable, and is probably rather better as one will not be fiddling about with a plug to get the light on at night. True it is not ideal, and may bring up bonding issues, although external metal pipework is unlikely in a shed. Fitting a second CU is not a requirement, although you may like it and feeding a distribution circuit from an MCB is not ideal either, and the cable needs RCD protection if hidden anywhere, so again, why not a spur? The 100m2 "rule" does not exist, and for a shed the volt drop makes very little difference to most uses. If the RCD is tripped from the shed it may be slightly inconvenient, but this would matter little to most people. Some electricians may not like the "look of this", but then we work to the regulations don't we? A simple catenery, or SWA if buried are not a problem to the ring, and we can choose the cable size for volt drop if we like. Simple and straightforward particularly if diagrammed as is suggested in the OP. There is no requirement to only use 2.5mm cable for rings, that is just convenient and conventional with BS 1363 fittings. Appendix 15 is only informative, and not a regulation, but this often slips through peoples vision without notice.

Dennis, I am not sure that this is a "better way". There is no reason in the regs why I cannot put a spur to my shed, whether it feeds a FSU and a light or not. It should be RCD protected, as I assume we are talking about the 18th edition. It is no different to using an extension cable, and is probably rather better as one will not be fiddling about with a plug to get the light on at night. True it is not ideal, and may bring up bonding issues, although external metal pipework is unlikely in a shed. Fitting a second CU is not a requirement, although you may like it and feeding a distribution circuit from an MCB is not ideal either, and the cable needs RCD protection if hidden anywhere, so again, why not a spur? The 100m2 "rule" does not exist, and for a shed the volt drop makes very little difference to most uses. If the RCD is tripped from the shed it may be slightly inconvenient, but this would matter little to most people. Some electricians may not like the "look of this", but then we work to the regulations don't we? A simple catenery, or SWA if buried are not a problem to the ring, and we can choose the cable size for volt drop if we like. Simple and straightforward particularly if diagrammed as is suggested in the OP. There is no requirement to only use 2.5mm cable for rings, that is just convenient and conventional with BS 1363 fittings. Appendix 15 is only informative, and not a regulation, but this often slips through peoples vision without notice.