I used to have a Tom Senior M1 mill with the standard vertical head,good well built solid machine,good for horizontal milling,unfortunately there is not much room under the vertical head,especially if Clarkson auto lock chuck is used, it would be ok if the E type head was fitted,lot more room and has a quill feed on the spindle though these are rare,though they do come up occasionally on E bay. you need room at the back to change belt speeds and remove the draw bolt for the horizontal arbour. Went to the factory once back in the seventies with a mate who collected his new E type mill,very good machine tool builders.
The Elliot omnimill looks ok plenty of room under the spindle though probably not so rigid as a Senior, Have a look at a Harrison,similar in size to the Senior though there appears to be more room under the spindle as the vertical head attachment is set higher in relation to the horizontal spindle, Some types of Senior and Harrison were supplied only as vertical,its far better to have the horizontal and vertical options. These mills are ok for small model building,i.e. 2'' traction engines,and 5'' locos,but if you may want to restore old motor cycles or stationary engines then a bigger mill is better i.e. Bridgeport as you always run out of room on a smaller mill.though a smaller mill is far better than no mill.