The supermodel and former girlfriend of the actor -- who's in the middle of waiting for a verdict with his other ex, Amber Heard, here in the States -- paid a visit to London's Royal Albert Hall Tuesday night ... where she watched JD shred on his guitar with Jeff Beck.

There were rumblings earlier in the week that Johnny had invited KM to catch one of his gigs while he was across the pond -- and wouldn't you know it ... she took him up on it!

You can see Kate heading out after the show Tuesday in photos obtained by DM -- where she seemed to be by herself with just a bodyguard in tow ... no sign of Johnny or really anyone else around her as the night wrapped up. Still, it's fascinating to see her out here.

Of course, Kate testified on Johnny's behalf last week during rebuttals -- this after Amber had invoked Kate's name the week prior when describing a fight she had with JD in 2015.

Johnny's team later got Amber to admit she only said that based on a rumor she'd apparently heard -- one in which JD supposedly pushed Kate down some stairs while they were dating in the '90s -- but KM herself shut that down and then some with her video deposition.

Not only did she deny that ever happened ... she actually made Johnny sound fantastic by saying he came to her aid and cared for her after she'd injured herself at the time.

It was pretty incredible that Kate was willing to do this for Johnny after all these years -- in fact, her doing so actually sparked online re-shipping between the two from fans online ... who wanted to see them back together, and who felt there was still something there.

We thought that sounded silly when it'd bubbled up on the bird app -- but after this, we're not so sure ... maybe there is actually an old flame that could be rekindled at this point.