No. Sitting is not an act restricted by copyright in the UK.

  16 The acts restricted by copyright [are]:
  (a)to copy the work;
  (b)to issue copies of the work to the public;
  (ba)to rent or lend the work to the public;
  (c)to perform, show or play the work in public;
  (d)to communicate the work to the public;
  (e)to make an adaptation of the work or do any of the above in relation to an adaptation

Source: CDPA1988 s.16(1).

Photoing a designer chair comes under copying: "copying includes ... the making of a copy in two dimensions of a three-dimensional work" (s.17(3))

IANAL.