An opening offer low £x,xxx is encouraging, if it is genuine. A keen buyer might pay more. Perhaps ask for double, and go from there. You can revert to opening bid if that is their limit, and you are willing to accept.
I recently had £2k offer for good a domain. Sender insisted on phone negotiation, then said they had sent similar offers to other sellers just to get a response. Their max spend would be mid £xxx.