Hello Flag UK,
https://support.brother.com/g/b/producttop.aspx?c=us_ot&lang=en&prod=hl2135w_us_eu_as
As you have discovered, macOS 11 Big Sur is not mentioned under Downloads.
Also, nothing about the HL-2135W here:
https://support.brother.com/g/b/oscontents.aspx?ossid=3
The HL-2135W is not an AirPrint printer:
About AirPrint - Apple Support
Since there is not any information from Brother about plans to offer macOS 11 Big Sur drivers for the printer in question (as far as I can see), you will probably have to rely on others to suggest workarounds. You could of course check whether an existing printer driver would work (may be limited by the operating system's built-in security technology).