You can't make that redirect conditional, but as also the Let's Encrypt documentation explains here, you can use DNS CNAME
records or NS
records to delegate specific records (and answering the DNS-01 challenge) to other DNS zones and/or servers.
That is a perfectly sound solution in case your current DNS provider doesn't provide a DNS API for automation and you cannot switch completely to a different provider.
_acme-challenge.example.com. IN CNAME example-com.example.co.uk.
where you use a single record example-com.example.co.uk. in TXT "challenge"
in an existing DNS zone or
_acme-challenge.example.com. IN NS example-com.example.co.uk.
where you set up a completely new zone that is authoritative for *._acme-challenge.example.com.
including _acme-challenge.example.com.
itself.