A construction company is not bound by the law to carry out an archaeological assessment of an area, it is only advised (PPG 16). If something is known about or turns up whilst carrying out the construction then archaeologists should be called in but people generally dont unless they are interested in history, someone complains to the council or human remains are found. A desktop assessment is usually carried out on supposed sites i.e where someone thinks there is something of archaeological interest and can then end there. If something turns up then trial trenching or archaeological prospection should be carried out, but again it is upto the construction company to foot this bill and something as large as the Channel Tunnel would have been forced through without too much concern.