Originally Posted by Lozek
I don't think you'll get what you want in a pedal form factor. It's definitely convenient though. I made the mistake of sticking an older non rack mount Nady on my pedal board precisely once, thinking, "it's only a little bigger than a pedal". It lasted for a grand total of 3 and a half minutes before one of the guys backlining in the headliners gear kicked one of the antennas off.
Dunno about interference, UK and US regulations are probably different, but a lot of people are switching to older units because the bands for most modern wireless units are so congested. Picking up an older premium Nady is usually a really good bet. Honestly, the construction on the older units, especially the transmitters, is usually better.
Some of the best ones you can get in the US are actually the ones they sell to public schools and universities in bulk for teachers to use. Not only are those usually solid as shit (if tax dollars are paying for the shit and the school is in a rich area), they are usually on less congested bands. Bands are always getting re-assigned and shit, especially in the US, so you have to do the reading if you don't want interference. I think the US is getting another band shuffle up courtesy of the FCC in 2020.
Your other best bet is just finding a solid one that uses the same bands as the preachers use in Mega-churches.
No joke. Any wireless lavalier one can usually carry a guitar signal in theory, although they obviously have ones especially marketed towards guitars. Churches are obviously the most anal about having frequencies that aren't susceptible to interference.
I believe Audix does some that might be close to what you are looking for. https://audixusa.com/docs_12/units/AP41_GUITAR.shtml