Micro:bit LED control using P30N06LE
You're trying to control the current through an LED strip by using an NMOS transistor. This solution can work (somewhat) but has several issues:


It is difficult to control the current, the NMOS is designed to respond "abruptly" to a change in \$V_{gs}\$. This is because these NMOS are designed for switching applications, meaning on or off. What you need is something between on and off, more like a resistance.
In the "resistance" mode you need, it is very difficult to keep a constant resistance. The resistance is very dependent on temperature. When used as a resistor the NMOS will heat up changing its resistance, which is what you're trying to keep constant!
For light loads this scheme will be OK, the NMOS will not warm up too much. But when the current increases more heat will be dissipated and keeping the NMOS cool can become an issue.


Fortunately there is a solution which solves most of the issues: PWM (Pulse Width Modulation). With PWM we switch the NMOS transistor on and off so quickly that human eyes cannot notice it. A PWM frequency above 200 Hz is often sufficient.

Due to the fact that the NMOS is either fully on or fully off, it will not dissipate much power so a high current through the LEDs is no issue.

Also dimming can be smooth as a 1 % PWM signal switches the LEDs on for 1 % of the time and off for 99% so you really get 1% of the full brightness.

As the Microbit contains a micro controller, it should be able to generate such a PWM signal and quite accurately as well.

You can re-use the schematic you already have. I am too unfamiliar with the Microbit to tell you if you can use the same output you're using now but chances are that you can. You might just have to set it to "PWM" instead of "analog" or "DAC mode".