Here's a section from the Wirral borough council website about metal detecting on the Wirral.....
The Council does not allow metal detecting on any of its land, including the Wirral foreshore. Metal detecting is illegal within the site of a scheduled ancient monument. Detecting on private land requires the written permission of the landowner. Any finds discovered in England are subject to the provisions of the Portable antiquities scheme and the Treasure Act 1996.
More information on the correct use of metal detectors is available via the Council for British Archaeology (CBA). The CBA also host the national Young Archaeologists Club.
The Portable Antiquities scheme is still administered by Liverpool Museums.
So basically, you need permission, or your own land to detect on.
Protection and Development
Wirral contains 9 Scheduled Ancient Monuments (SAMs), which are regarded as being of national importance.
1. Birkenhead Priory
2. Bromborough Hall moated site
3. Grange Beacon
4. Irby Hall moated site
5. New Hall
6. Overchurch site
7. St Barnabas Cross
8. Standing Cross at Holy Cross (Woodchurch)
9. Storeton Hall
Scheduled Monument Consent is required for any works to an SAM, including maintenance, alteration or development, and even for below ground works to a standing monument. Applications are made directly to English Heritage.
The protection and conservation of archaeology is a material consideration within planning law, and the legal provisions are enshrined within Planning Policy Statement 5 – Planning for the Historic Environment, which introduces a new holistic approach to conservation. All development works to an ancient monument, scheduled or not, requires planning permission – there are no permitted development rights for protected monuments.
Undesignated archaeology within the Borough is protected under the provisions of local policy CH25 Development Affecting Non-Scheduled Remains.