I hate having to defend MPs, but - Ministers are the ones that should be staying at home running the country. They should only be travelling away when it is clearly in the furtherance of their ministerial obligations.
Being an MP is about more than being a representative for the constituency who elected you. That is a significant part of the job, but much of the work of the House of Commons is done by Select Committees, who scrutinise issues and examine and test legislation. These are staffed by backbench MPs. MPs are also expected to vote on various issues put before the house, so although one might expect them to vote with their party, it is also important that they have spent time and energy understanding issues which will have an impact on their own constituency.
All of that notwithstanding, trips abroad should not just be seen as a perk, a freebie for the elite ruling class, nor should they be allowed to participate in such things without declaring they have and declaring an interest in any subsequent committee they serve on or report they author.