There is in general no maximum multiplexing frequency.

The limiting points are only EMC considerations and the speed of your driver stages (transistors).

Just what I wanted to say, Peter!
Red LEDs show a turn-on time in the 10...100nsec range, if I remember correctly. High efficiency types, can show some relevant junction capacitance, which is sometimes specified in datasheets of single LEDs. I think something in the range of 50...100pF has to be taken into account.
From these data you see, that theoretically some 10kHz should easily be achievable. Even time constant resulting from current limiting resistors in combination with junction capacitance does not worsen situation.

Situation is a bit different, if you look at the multiplexing driver. If standard transistors are used, turn-on and turn-off times can be relevant. In any case, these transistors are very probably slowier than the LEDs, so, these will limit maximum multiplexing frequency.

As you live in the UK, your product has to comply with CE standards! And if you have fast edges, you will get lots of troubles with radiation. So, 'slowly' standard transistors can be very helpul in limitting radiation. Unfortunately, display cannot be shielded by Faraday cage, of course.... So, a low ration level is very important.

I would recommend you to use at least double sided PCB, with massive ground plane, which at least covers copper traces to the LEDs. This can help to minimize radiation loops, and by this help to fullfill CE standards.

Good luck,
Kai