If it were me,
I'd pull the plugs and hope to find the piston with the clean top. That will show which head is cracked. A rebuilt from the US is $1900 USD. Divide by 1.7 for Pounds. Dump the oil and coolant. Dump the cracked head. Replace the cracked head, oil and coolant. Drive on.
If no clean piston top is found then either a head is not cracked, or one's cracked and leaking directly into the oil, so troubleshooting gets complicated and decisions get harder.
The downside with simply replacing a head is IF the internals are damaged, they may bite you later. The upside is, if they are ok, which they well may be, it's the cheapest fix going.
