You'll need to obtain an
independent valuation (which rules out simply submitting your own figure). HMRC advise that an under-valuation by an estate agent could lead to the executors of a will facing penalties but such penalties would not be charged where the method of valuation was 'reasonable and proportionate'. Here's what the Society of Trust and Estate Practitioners advises its members:
http://www.step.org/probate-valuation
Remember that 'estate agent' is not a 'protected title' in the UK (unlike in many other European countries). Anyone can call themselves an estate agent, so you'd need to ensure that any valuation you got from an estate agent came from someone who could show that he really knew what he/she was doing (e.g. through membership of a relevant professional body).