Collecting / returning loaded or empty trailers (remember Booth vs DPP defined an empty semi-trailer as a load, as it imparts a burden on the drawing vehicle) from UK ports you only require a Standard National op licence. However, your vehicle may not leave the UK. The really confusing one is that a company operating on a Restricted licence (carrying their own goods) can run abroad quite legally provided they carry only their own goods and do not carry out any hire or reward work.
Be really careful with CMR insurance. The whole journey will be under the CMR limits (much higher than FTA/RHA limits), no matter what your ‘terms of business’ say. I know a lot of hauliers who have got badly caught out by a CMR insurance claim