No, it's not advertising, it's maintaining a high profile, which increases donations massively. Is that not a good thing?? You do realise that charities increasing donations is positive? It's not giving money to shareholders, it's providing vital services to help people!
There do seem to be swathes of people who think attempts by big charities to..um.. raise money are somehow underhand or Shouldn't Be Allowed. They are not multi-national conglomerates trying to maximise profits for shareholders through devious charging practices or tax avoidance. They are spending their money on either services of some type for people in need, on raising money to provide those services, or on administration to ensure both those things happen smoothly effectively and efficiently.
Paying for adverts on telly or similar is different, although charities wouldn't do it if it didn't raise income - they aren't (mostly)actually daft.
But keeping the Macmillan name on services initially funded by Macmillan is a no-brainer. Brand familiarity encourages people to access the service, and also encourages them to donate, thereby increasing service-provision. If the nurses and other services had no link to Macmillan and were "just" seen as an NHS service, people feeling grateful for the service wouldn't be donating to the NHS instead.
Charities big and small have an obligation to spend their income in a certain way. Attempts by charities to increase their income, as long as they are not ethically dubious or unlawful, are to be encouraged. Until the government funds research or support services massively more than they do, these charities are essential and make a huge difference to many lives.