Which lens filter brands do you recommend for protecting Leica glass?

Hey, totally get why you're ditching the cheap stuff. Putting a basic filter on a Summicron is like putting budget tires on a Porsche, it just doesn't make sense tbh. I've learned the hard way that cheap coatings are basically flare magnets. Since you're heading to London and want to keep that contrast sharp, I would suggests looking at these:

• B+W 39mm 010 UV-Haze MRC Nano is probably your best bet for the money. The MRC nano coating is way easier to clean when you get rain or smudges on it, which will definitely happen in London weather. Just be careful with the fake ones on Amazon, make sure to buy from a reputable photography shop because the knockoffs are everywhere.
• Hoya 39mm HD3 UV Filter is another solid choice if you want something really tough. They use hardened glass so if you're worried about bumping into things while sightseeing in crowds, this one might save your lens.
• Leica E39 UVA II Filter if you just want to stick with the brand. They're great glass, but honestly you're paying a huge premium for the logo and the peace of mind. One thing to watch out for... make sure your lens hood still fits over the filter. Some of those thicker frames can be a real pain with the Summicron hoods. I usually stick to the B+W slim versions just to be safe. You really dont want to spend all that money on a trip and find out your corners are vignetting because of a bulky filter ring. Also, be careful shooting straight into streetlights at night, even the best filters can catch some ghosting tho.