From UNCTAD, as of 3 Sep 2019:
Among the countries which currently grant preferences to the EU but for which the UK has not yet reached an agreement to grant continued preferential market access, Turkey, South Africa, Canada, Mexico, Japan, Egypt and Morocco are the markets where the UK is expected to have larger export losses. In particular, the UK is expected to lose about USD$500 million of exports in the Turkey market, about 5 percent of its exports to Turkey. In South Africa, the UK is expected to lose about USD$240 million, equivalent to about 9 percent of its exports to South Africa.
Even more recently, on Sep 11 the UK announced an "agreement in principle" with South Africa and a few other smaller [economically at least] African countries (but that doesn't include Egypt or Morocco.)
But what do we know of the UK-Turkey negotiations? (According to UNCTAD, Turkey was the largest non-EU trading partner that has an arrangement with the EU, but insofar not with the UK in the post-Brexit horizon.) Are there any sticking points in these UK-Turkey negotiations that have been made public?