This Biologic Postpump 2.0 would make a fine albeit somewhat heavy 718g baton/seatpost/pump, 580mm long. At 33.9mm diameter a big large for small hands though.

If you cut off the seat clamps it'd be an awesome personal carry. Would still be a legitimate personal carry perhaps even in the UK if you're cycling.

http://www.thinkbiologic.com/products/postpump-seatpost

Recreating it as a frame pump might require creating some custom pegs for your frame, and modifying the sawed-off (ex-seat clamp) end of the pump.

Just ordered one, so I'll soon know all. Might have a machine ship create some knurling at the end under the seat clamp, the handling end for defense purposes.

Biologic also makes some short ones (eg 350mm) in other diameters like 27.2mm and 31.6mm. Bit short to call a baton at that length though.