Waze, a free smartphone nav app, offers the capability to offer a route that will avoid the ULEZ. I use it myself so can vouch that it's great. If you have a smartphone, you should be able to buy a phone holder for your car and then you can use it legally so long as you don't touch it while you drive. It's more useful than a lot of aftermarket navs, in my opinion, because you can see real-time data inputted by other road users. The only catch is you need a smartphone and you'll have to use your own data — which can be pricey depending on what data plan you have.