It would be a pity to lose these comments as they have provided interesting reading. I will not pretend to understand the fine print of STGO regulations and Construction and Use, which seems to change depending on what you call something.
I must admit that when you do the maths it must be a pretty close call.
If the Milly is 21 ton, the trailer is 8 ton and the Valentine is around the 16/17 ton mark (according to Wicipedia) it is well within the stated train weight of 60 ton but the axle weights for the trailer must be tight, 8 + 17 = 25. 25 / 2 = 12.5. If the Trailer is only rated at 10 ton or even 14 ton load, the Valentine would be over at 16 ton.
As to widths I assume that if the Milly is classed as a Locomotive when pulling a trailer the 2.75 meter limit comes into play?
That said, from the pictures of it loaded it looks ok. ie it fits on the trailer and nothing looks overloaded or insecure so would be unlikely to attract the wrong type of attention. It is only likely to cause a problem when something goes wrong, either mechanicaly or due to other road users, hence the importance of making sure everthing is in order.