I have no opinions one way or another about using the builder's mortgage broker other than they all get a share of the commission generated. At the end of the day, if you are happy that the mortgage you are offered is the best deal for you taking everything inot account including arrangement fees, then I cant see that using a broker can do much harm.
Thankfully, you have saved yourself by not using Redrow's preferred solicitor but to be sure you should as for their list, just to be sure your one isn't on it!
You are right that Redrow's year-end is 30 June and that it is always best to avoid buying any new home due to be completed eight weeks or less before the year end date. However, you should be equally mindful that a completion date could easily be brought forward and your home rushed even more.
I would suggest you get your solicitor to inform Redrow that the earliest you can legally complete is Friday 7th July 2017. In addition, 1st July 2017 is a Saturday and most solicitors will be unlikely to be working to transfer mortgage funds on a Saturday.
So I expect that your legal completion date is in fact Friday 30 June 2017, the latest day that Redrow can leave it for the current year-end and that your home could well be a rush job.
With five months to go, there is plenty of time to complete your home depending on what stage it is currently at.