What focus and what kind of techniques are different?
Krav-Maga is close-combat. Originated by Imi Lichtenfeldt, however, as to my knowledge there are plenty of organisations and derivates existing. The difference is who is teaching it, as there is no restriction to anybody to call his business in any way except there is some kind of trademark set. The main problem with this is that anybody who was in the IDF or participated in an Krav-Maga a instructors course can open a Krav-Maga school and give it his own name and apply techniques he see fit his 'business' or current needs.
My advice, stay with its roots and train in gyms that are part of quality controlled organizations like KMG, IKMF or KMA then it doesn't matter how the instructor calls it, it might be cake-krav-killer-defense but you will have at least some sort of confidence that the things you get taught have part of the idea in it of the originator.
The differences between Krav Maga (KM) and Urban Krav Maga (UKM) are probably moot if you are training to be effective at self defense in general. I trained in Urban Krav Maga (North America) in Madison, WI for 4 years before a job moved relocated me to Chicago, where I now train at a Krav Maga Alliance school (Krav Maga Force). I also spent 3 weeks at Krav Maga Academy in San Diego training with them a few years back - so it seems fair to say I can provide some perspective.
Like many other answers to similar questions, the training depends heavily on the instructors priorities and their backgrounds. The affiliation (e.g. KM Alliance, KMWW, etc.)provides some guidance to the scheduling, techniques needed to pass testing, and how students increase in rank. Urban Krav Maga was, as I learned, originally affiliated with Commando Krav Maga (CKM). When the CKM founder, Moni Aizik, was discovered to have embellished his credentials, there was a bit of an uproar and some distancing from his CKM school. His techniques were not necessarily bad, but the name had been socially 'poisoned' to some extent.
How UKM-North America (UKM-NA) and UKM-UK might differ is less clear to me. We followed the same basic curriculum with perhaps more emphasis on the types of knife attacks, and attacks in crowded spaces (such as trains and buses) in the UK, and more emphasis on gun defense in the US. Testing up a level had the blessing of UKM founder Stewart McGill (with his signature on the certificate).
Minor differences between UKM and KM I have observed include: UKM-North America emphasized the palm strikes and (vertical) stacked knuckle punch form, whereas the Krav Maga Alliance instructs their students to follow a 45 degree punch (they say it is closer to the relaxed position and thus more 'normal') to minimize metacarpal damage; there seemed to be more emphasis an precision and technique (with more practice dedicated to building muscle memory) with regard to wrist locks at UKM-NA, and less of that at the current KM school; more aggression at the KM school compared to UKM-NA. The gun defense at the current KM school follows the traditional KM techniques, but at UKM-NA there was more emphasis on getting two hands on the weapon hand quickly. KM has more light sparring with 16 oz gloves whereas UKM-NA had occasional light sparring without gloves. So overall - there are small differences which allow the more experienced practitioner to easily adjust and accommodate changes - but there are no major differences as far as I can see. From my latest information, UKM-NA will shut its doors at the end of April, 2019 - due to retirement of the lead instructor.
I've benefited from seeing how different instructors and schools approach the 'self defense problem'. People often get taken in by their instructors approach, but it's good to expand your perceptions and accommodate new approaches. I'd advise others to try the same!