The design I ended up with is based on a Raspberry Pi 3, and uses an Adafruit PN532 NFC/RFID reader, a NeoPixel LED ring, a Grove relay from Seed and a couple of other small parts (see the full bill-of-materials on Github).

We can use a number of different RFID/NFC tags and cards as entry tokens, including (but not limited to) NFC Type 1 to 4, Oyster cards, contactless debit cards and Apple/Android Pay.

For the action of actually opening the door, I've modified the office buzzer system to allow operation via a relay, whilst the usual button press works too.

The NFC reader, LED ring and the relay are all controlled by the Raspberry Pi through the GPIO pins.

The whole setup is placed behind one of our front windows and a poster, so it's not visible from the outside, but tags and cards can still be brought within reach of the reader.