The Civil Procedure Rules, Part 31, describe, amongst other things, the duty to disclose documents. (definition of a document available here)
However, this disclosure can only occur if there has been an effective search of the documents. Section 31.7 of the CPR defines the duty of search as:
Duty of search
31.7
(1) When giving standard disclosure, a party is required to make a reasonable search for documents falling within rule 31.6(b) or (c).
(2) The factors relevant in deciding the reasonableness of a search include the following –
(a) the number of documents involved;
(b) the nature and complexity of the proceedings;
(c) the ease and expense of retrieval of any particular document; and
(d) the significance of any document which is likely to be located during the search.
(3) Where a party has not searched for a category or class of document on the grounds that to do so would be unreasonable, he must state this in his disclosure statement and identify the category or class of document.
The key factors here are the scale of the review and the size of the case, in short it comes down to proportionality
If there are 20 million documents available for review (which in modern email systems is entirely plausible), and the case is worth £10,000 it would not be reasonable to search all of these documents, especially if the documents were stored in a complex and hard to retrieve manner.
Equally, the reverse is also true: If a case is worth £1.8 million, and there are only 10,000 documents to review, then it would be expected that all of those documents would be searched.