I have experienced both a cross curricular Humanities and a subject led approach to Key Stage 3. Particularly with students who have struggled at Primary school, there can be a benefit to a thematic Humanities approach in Year 7 as a form of transition to the secondary curriculum. It allows a bigger story for the students to connect with, more project based learning and can also provide more continuity if one class teacher covers each subject strand. However, it can be very difficult to make good progress in the individual concepts and processes of each subject, particularly if students are already working at Level 4/5.
Teaching thematically requires a great deal of planning to make it effective. There also needs to be a strong link between the subjects either as a narrative or concepts or processes. Examples of things that I have found worked well are:
narrative - Pompeii, volcanoes, Roman Gods, early Christianity and modern beliefs
concepts - industrialisation and settlement
processes - using evidence to explain different views
Very often the pressure to teach thematically comes from SLT as a response to student disengagement with the subjects, or simply as a way of easing a timetabling issue. What I think SLT fail to appreciate, is that when a thematic approach is not properly planned it is considerably more prone to disengage students. As soon as you find yourself asking how you can make something 'fit into the theme', you can guarantee that it won't be long before the students are asking 'why are we doing this, I though we were looking at X'! As a result, you may wish to teach your Year 7 Humanities course as separate subject units, or perhaps a mixture of the two.