A price to pay for having access to such a wonderful building. You should check London Open House; release dates and booking normally starts in August for one weekend towards end of August early September. Access if you can get it, is free to many buildings.
Bit difficult from the top of the Shard. The opening at the top of the glass is at least 10 foot from the floor.
The experience for want of a better word, is much better at the top of the Walkie Talkie and better views.
The very best views are from the top of the Post Office Tower; if you can ever find it's open