Hi Jamie,
A few things spring to mind that you may wish to consider:
1. Profit share - you don't have to fix your profit share percentage within your partnership agreement (such as 50/50 or 60/40 etc), you can have a phrase such as "profits will be split by agreement between the partners according to performance" or something similar. That way you can retain a degree of flexibility in allocating profits between you. (E.g in one year perhaps your wife may have other income and it would be beneficial for you to take most of the profit from the partnership, the above gives you the mechanism to achieve that).
2. AIA - the Annual Investment Allowance: This is relatively new rules regarding how equipment etc that you buy is treated. In essence the first £50k worth of equipment (per year) is written off in full against your profit. This can bring significant advantages where, for example, equipment is bought on HP or similar finance. Take for example a new van bought for say £20k (on HP) on the last day of your trading year. You would get full relief for the whole £20k in that year even though you will be actually paying that sum of money over the term of the agreement (perhaps 3 - 4 years?). Its one of the few times where you can obtain tax relief in advance of actually spending the cash!
3. 1st year losses - if you purchase a lot of equipment in the first year then the AIA relief may actually reduce your profit into a loss. If that occurs make sure that the loss is offset against your highest taxed source of income possible. (So for example if either your wife or yourself had a job where you have paid 40% tax, the loss can be offset against that employment income and the tax reclaimed).
4. Tax credits - Don't forget about tax credits. If your income is low in the early years of trade you may qualify for tax credits, its certainly worth checking
5. Claim all legitimate business expenses and keep good records (obvious I know!)
Hope there's some useful pointers there!
Best wishes
John