Firstly, the rental profit will be lower for US purposes than for UK purposes, because the US requires that your client claims depreciation on the building (but not the land) on a straight line 40 year basis.
Secondly, you do not state the form of the UK business. Is this a sole trade, a Limited Company or some other entity?
Thirdly, FBARs and 8938s and PFICs (including in ISAs) are frequently overlooked.
Fourthly, does your client have any carryover on Form 1116 from the previous year? If 2015 was the first year of residence, is this a dual status return and - if so - are you reporting the foreign income only for the period of US residence?
PS – Greeting from London, England. Grey and rainy ...