Assuming you want only to transit the Class D airspace, you do not need a transponder at all (common misconception) – it helps a lot but it’s not a Requirement. Your Mode A should establish identity with no difficulty. However, also assuming that you haven't been 'handed over' and it's a 'Free Call' it may assist, when invited , that you check in with an accurate position report, eg: “G-ABCD, LAM (or wherever - BKY?) ABC radial, XYZ DME tracking MNO, 2500ft on the Luton QNH requesting……..”
Last edited by Downwind.Maddl-Land; 14th May 2014 at 22:21.