From aa-asterisk.org.uk wiki
A DE block is the term used by Oftel originally, and subsequently Ofcom, to describe a block of 10 000 telephone numbers, which was the minimum allocation back then. The D and E digits are the fifth and sixth digits of a UK telephone number, excluding the initial 0 trunk code or the +44 country code. For example, 0123456xxxx would be allocated to, say, BT. The underlined digits are known as the D and E digits, and therefore a block of numbers was known as a DE block.
These days, in conservation areas, numbers are allocated effectively as 'DEF blocks' (but are not known as such), and in non-conservation areas typically an operator is allocated a DE block with the stipulation that only a certain F-Digit is to be used. If that operator then runs out of numbers further F-Digits in that DE block are allocated to them until all F-Digits are allocated. There is no longer any requirement for a DE block to be allocated to a single operator.