Hi
some of your requirents are kinda vague, what is a messenger bag? and different brand of bags vary in sizes. Because you can get 12" android tablet from Samsung if you really wanted to go that big. But i would assume 9-10" is more than enough for you.
and do you really need the tablet to have mobile sim card slot, would it not be better to tether to your phone? It would save you some money and open up some more options.
but to meet your original requirement, you could try a Sony Xperia Z2, 10" screen and comes with LTE so inset sime for mobile data usage and very good tablet. There is also a 8" version Z3.
There is also Galaxy tab S 10.5 with 4G and WIFI which comes in at almost £500. There is also a 8" version of the tablet S 8.4" iwht 4G.
Not much else i can think off right now, there might also be a lenovo android tablet and maybe ASUS made tablet but cant remember to be honest right now. The ones I have listed are the best quality wise specialy the samsung has amazing AMOLED screen, have seen some chinese tablets with your requirement but stay away from anything that is not from big reputable company otherwise you will trying to fix it for ever with updates, bugs and patches if you are lucky enough to get them.
