ULez is London only, and covers the same area as the congestion zone. if your vehicle is not Euro 6 (check registration doc) then you must pay the daily fee of £12.50 for small vans. Charge applies on all days, incl public holidays, and is in addition to the congestion charge.
It will later (probably 2021) be expanded to Greater London.
Enforcement is by ANPR camera (as with congestion charge)
Other cities will be introducing Clean Air Zones which are almost identical, requiring EURO 6 compliance to avoid fees, but the fees have not yet been finalised. These will start in 2020 or 2021.
Euro 6 diesel vehicles have an SCRT system. Basically, after the DPF, AdBlue is injected into the exhaust stream over a catalyst to convert any harmful NOx compounds to CO2. So if you have a vehicle that requires AdBlue it is almost certainly Euro 6.