This subject comes up on Mini forums a lot. The A-series block is supposed to have all British threads which would/should make that hole 1/8 BSP... which is slightly smaller and has a different pitch than 1/8 NPT. Nonetheless, I've always used a 1/8 NPT compression fitting to 1/8" tube in that hole without issue. I buy my generic gauge line kits from Advance Auto.
An additional thing to keep in mind is that between the firewall and the block you should provide at least one loop about 2" in diameter in the tubing to absorb engine motion. I usually use at least two loops forming what looks like a spring made of tubing. The nylon tubing kits don't require this but I don't use them anymore after bumping an old line, cracking it, and quickly pumping a quart of oil on the garage floor.