Unfortunately many claimants have fallen foul of this - the enhanced rate mobility on PIP is just tougher to qualify for than the high rate mobility was on DLA - this is one way the government plans to save money on the welfare spending. Crazy.
It is unlikely that the Tribunal's decision would be further challengable, unless you think that the Tribunal may have made an error in law, for example by misinterpreting the legal test for the benefit, or disregarding relevant persuasive evidence - it's fairly rare. The fact that you dispute the outcome is not sufficient ground for further appeal.
Perhaps a better bet as CR alludes to is to try again for the enhanced rate mobility on renewal of your claim next time round.
Sorry that I'm not the bearer of more positive news - were there any noticeable errors in law? If you think it's possible you can ask for a Statement of Reasons from the Tribunal within one month of the decision, and can then have a really good check if any errors were made (it's best to try and find a local specialist, Citizens Advice/ Welfare Rights/ Law Centre if you can).