Usually sign on bonuses, if not offered in advance, are only negotiable if you have a good argument for them. A typical one I have heard for experienced hires is that "if I worked at my current company until Bonus time, I would get a bonus of £10k. Therefore, if you want me to start now, I would need a £10k bonus to compensate". You could try this, depending on how early they want you to start.
As for tenure - you can probably ask what the possibilities for fast promotion once you join. However, with only 2 years experience, it's unlikely they would offer you anything higher than a Consultant position. Most people I know that did switches like you started at the same / lower level than what they were at their previous firm, but then performed strongly and were able to get promoted much faster than the regular track.