The frequencies or bands of cellular telephony used in the United Kingdom are the following:
2G/GSM: GSM 900 MHz, GSM 1800 MHz.
3G/WCDMA: UMTS 900 MHz, UMTS 2100 MHz.
4G/LTE: LTE 800 MHz, LTE 1800 MHz, LTE 2600 MHz.
You can check if the Astarry Sun 1 works in the United Kingdom since it has a connectivity 2G:
A 3G connectivity:
4G / LTE connectivity: