Despite the Housing White Paper and even legislation, like the Housing and Planning Act, legislation change seems to be moving slowly. Yes there are plenty of hurdles to cover and indeed we would much rather have "right" legislation, carefully thought out, than "fast" legislation, so full of holes that if has to be edited. However, with the focus on Brexit and the weakness of a coalition government to boot there are real problems changing laws.
The normal policy is to only introduce new legislation at the beginning of April and October each year unless there are extraordinary factors (like urgency or EU commencement dates). The law needed for the tenant fee ban is, apparently, being drafted if the Housing Minister is to be believed. However, they say the delay is partly caused by over 4,500 responses to the consultation which they have to work their way through. In fact if they were meaningfully working through responses one might have hoped that they would NOT be drafting legislation but waiting till they have read the consultation responses first?
In Scotland the legislation was in force from 1984 and the 2012 legislation was simply clarifying what was already in place. For England (Wales will pass its own legislation, which is also being consulted on) it will require primary legislation (an act of Parliament). This receives full Parliamentary scrutiny meaning that it can take quite some time to get through, depending on the size of the bill, how controversial it is and the size of the government majority. Whilst the government majority is weak the bill is not likely to be controversial across the major parties and whilst it will take time to "process" it is not likely to suffer undue delay. Crucial to the speed through the system will be whether it is in a bill on its own or it is part of a larger bill that may contain more controversial elements in it. It might be in for April, but this would be quite swift, and October 2018 is probably the best guess.
There is another reason why October 2018 would be a "good" date for it to start, and this is advanced payments. If you do student lettings then from October this year onwards you might start letting properties and charging tenants fees, perfectly legitimately. If the new legislation in introduced in April 2018, what would be the position of all the fees paid before the ban for agreements starting after the ban? Whilst this could be a massive problem for student letting providers it will affect all agents to some degree unless enough warning of the implementation is given and having clear transitional provisions will be important if we are to avoid having to refund money.