"XYZ radar, GABCD 10 north with zulu for landing"
"GABCD XYZ radar roger, QNH1013, Expect runway 26 report airfield in sight"
"GABCD wilco"
Sorry, but in my view this is lax and lazy controlling. In the UK, once you have left controlled airspace you are not entitled to re-enter unless specifically cleared to do so, unless there are standing agreements between your flying organisation and the ATCU controlling the airspace. If such agreements were in place I would expect there to be conditions applied, such as a specific route to follow, and/or maximum level. If this clearance is not forthcoming, then as others have said, it is best to clarify that you have clearance when you read back. Unfortunately, a simple "wilco" is not enough.
Also under a FIS, you are not under the control of ATC, you are, as the name implies, in receipt of an information service only. You do not need to ask for any changes of level or heading. You are not required to make any reports, unless asked to.