You will need some kind of local means of isolation here and not to a junction and or spur hidden in the rafters unless they are open exposed rafters with easy access to isolate the cable, also cable zoning will need to be considered here as well as protection with an rcd if buried less than 50mm which almost certainly will be the case so would the circuit have this protection.
What is the nature of the circuit you wish to connect to?
Does the existing circuit provide rcd protection?
Your addition to the existing circuit will need to be compliant to the 17th edition/ 3rd ammendments of the BS7671 regardless of when the circuit that you wish to add to was installed, in some cases this can mean upgrade of the existing circuit but not always - it is hard to give advice in the form of a yes/no answer without you providing a lot of info.
If you can provide info about some of the points and questions I have posed then we may be able to give a straight answer, if not it will usually come down to the same response which has your safety as priority and get a competent person in to assess the work needed, this will also include testing of the existing circuit you wish to add to to ensure that it is not in a dangerous or unsafe state, something that usually cannot be done on a DIY level due to the nature and cost of such test equipment.
Hopefully this gives a little insight into what may seem a straight forward task can be a mine field of regulations, sometimes we can give those quick easy straight forward answers and sometimes not dependent on the nature of the question you ask.