You could look at the Marshall Class 5. It looks amazing and sounds ok, it doesn't sound as close to a classic Marshall as I was hoping. But I don't think you're going to find a single-ended amp design (and the low wattage amps tend to be single end) that gets closer.
I would encourage you to research the various 18 watt Marshal amp kits/clones that are available. Whether or not you want to build an amp, the sounds that you can get out of that model are very close to a classic Plexi tone to my ears. Worst case scenario you'll have a reference for the sound that you're asking about from a "small" amp.
If that all sounds way too involved, look at the Orange Tiny Terror. Not Marshall but a great British tone from a small amp that you can pick up around $400 used (for the head).
Also worth mentioning that getting a Celestion G12H or Greenback will make your current amp sound more Marshall-y/British. The speakers play a big part in that sound.