There is nothing wrong with candidates and parties reaching this kind of agreement, however they need to observe the rules and ensure that any spending or donation given is properly reported.
Agents need to authorise candidate spending. Where notional spending is being incurred, the candidate or their agent must make use of it for it to be notional spending.
A registered political party can also be a registered non-party campaigner. A party who registers as a non-party campaigner will have to complete a separate return to report its non-party campaigner spending.
Registered political parties cannot donate, via notional spending or directly to a non-party campaigner. You must ensure that any donation you receive as a non-party campaigner is from a permissible source.