Transmit Issue
The init() function leaves a single byte in the FIFO transmit buffer which, as expected, is transmitted first. Not an issue with some peripherals it is vexatious with such as the Honeywell PM2.5 sensor - which only accepts fixed format commands and which seems unable to recover having received junk.

However, the transmit buffer problem is easily deal with - flush with a dummy read. Thus the PM2.5 sensor additional effort (eg a relay) appears essential to delay sensor power-on activation until the micro:bit's transmitter buffer has been flushed.

Receive issue
Most of the uart.receive functions appear not to work as specified - however, uart.readall() does what is required.

Question: have the uart.read.. functions been thoroughly tested, are they a work in progress - or, should I be back on the tablets? :)

On the credit side i2C and bluetooth BLE radio have been a joy to work with.