Hi, OP -
Congratulations to DD! That is a wonderful programme.
DD’s best sources of guidance for this question are the academics she is about to meet at Imperial. They will be more than happy to advise.
The worry is that her technical background won’t be sharp when she begins her PhD. I would expect that in this respect spending a year travelling would be looked upon quite differently to spending a year as an RA. In fact, I hear that in some very competitive life sciences disciplines Americans are enhancing their PhD applications by taking a year out to work as an RA. But they don’t have the worry of losing Maths skills, so I am not sure the parallel carries over.
I am in a strongly mathematical discipline and I worked in the Civil Service for two years between UG and PG (my MSc is rather incidental). But I was near a good university and able to attend one or two weekly seminars most of the time.
I really can’t say how the travelling would be perceived. I mean that honestly - I am not trying to sound negative. One thought is that if DD wants to see a bit of the world but is not committed to particular destinations, could she tour some good Physics destinations? Hang out for a while in Meyrin (CERN) attending lectures, maybe visit Boston (MIT and Harvard) and the Bay Area (Berkeley, Lawrence Livermore, Stanford) etc. Anywhere with good Physics - from Seattle (UW) to the beautiful beaches of La Jolla (UC San Diego), and that’s just the West Coast of America. Paris, Germany, etc.
Someone with an MSc from Imperial will be welcomed at whatever lectures she would like to attend, and if she has questions about the doctoral programmes there will be PG students happy to talk with her. There will always be PG students happy to talk with her!
She would need to make this a bit of a project, doing some independent learning on the topics she found interesting. Keep notes, etc.
When she makes her PhD application it will likely either be to some kind of doctoral training programme (there are different names) or possibly to a specific funded project. In assessing applications, the most difficult thing is to get a sense of student’s initiative, resilience and creativity. If someone discussed the kind of thing I’ve outlined above (from an intellectual perspective, of course) and explained how it led them to my programme or project, I would be impressed.
But mainly DD should follow Imperial’s guidance
Best wishes to her