The distributors of most US TV series don't release the international rights until at least 4 months after the programmes are first aired in the USA. (The Showtime online service, where Series 5 of Homeland can currently be seen, is only available in to US subscribers).
The first series of Homeland aired in the USA 4 months ahead of the UK. Series 5 first aired in the USA last October, so it's likely that the UK broadcasting rights will be released soon. As long as Netflix has won any bidding battle that there might have been for them (against, perhaps, Sky or Amazon Prime), you should find the programme available via Netflix in the near future.