I'm not sure so much can be done with the ressources of the time, and I'm even more dubious reading CaptainAmerica's idea of a western byzantine empire. The main reason for that being that Gaul really has left roman hands by this time and thus the path to Spain is wide open, as is the path to Italy. There is simply not enough forces, even with italian manpower, to man both sides of the Alps (western/gallic side and eastern/balcanic side, not even speaking of the northern invasion path through Helvetia) and the Pyrennees and keep enough forces to repel desert barbarians in Africa. Also communication between the three parts of this western empire (Spain/Italy/Africa) would be plagued by pirates from Southern France (a land that can't really be held without the Rhine) like what the Goth had done in the Egean earlier and what the Vandals would later do in our timeline.
Now if we only look at the suggested POD of WorldWarZ and have Aetius go to Britain, first we'd have to think about what forces he could count upon and what ressources he would find there :
- If he's fleeing for his life (taking what other posters suggested), he might not have a large military force under his command, maybe just next to 200 (which would still be a serious amount for the time, especially if they are disciplined veterans). Why not more ? Two reasons : troops might not follow a looser (and he would have few troops ready to follow if he just lost a battle as suggested by JosephK, especially since Boniface, also in search for troops, might offer a general pardon to the common soldiers of his ennemy, leaving Aetius with only his personnal bodyguard) and transport to Britain would lack to carry a much larger force.
- There he would find very few forces and a lot of empty fortress and marauding barbarians coming from the north or from the sea. He would have to first deal with establishing himself with a base of power in some city (probably a port in order to stay connected with events on the mainland, especially if he hopes to get back there), then secure the area around his base of operation to get some revenues allowing him to build enough forces to secure the interior of the roman province and put minimal maning at the most strategic places, thus providing conditions adequate for farming and, if possible, trade and industry. This would probably take him 5 to 10 years and leave him with the problem of sea borne raids.
From here he has two options : keep dreaming about the mainland and strike at Gaule, which by now will probably be an even bigger mess due to the fragmentation of power, especially if Boniface (or anyone else) was unable to win against Attila. An expedition would be costly and probably be over his capacities. Or, second option, he decides to make is fortress island his new kingdom since, after all, he's already alfway to making it viable and his rather organized and peacefull land would be the sole manufacturer of many products of high value that could be sold to either Spain or the gaulish kingdoms/areas/... His main targets would then be securing the seashores and the Hadrian Wall, with maybe the idea of going north of it to take preventive actions against those pesky northern barbarians. Also of interest to him might be to increase trade with Ireland to develop Britain's market.