well the money you make on buying and selling stocks comes under Capital Gains laws.
this means that the share sales are recorded on the Capital Gains pages of a Self Assessment Tax Return. any dividends from that you may get from and shares held for a period of time are also recorded on the form in a diff section along with any other sources of income such as a pension and bank interest received.
if you want to go legit, then you need to contact the Inland Revenue to request a Tax Return form, you will need you National Insurance number for them to set up an account for you.
go to www.hmrc.gov.uk for contact details.
now for abit of tax knowledge......
Capital Gains Tax (CGT) is payable on the gain made, less your annual allowances which is currently £9200 per tax year, per person (6th April to 5th April)
so for example, if you bought shares for £10000 and sold them for £12000 the gain is £2000 and is under the £9200 allowance, so no tax is payable.
if the gain was say....£10000, then you would only pay tax on £800 as 10000-9200=800.
now each person has an allowances, so potentially, you have £18400 worth of allowances between you and your wife. if you buy the shares in joint names, this would lower the tax due as you would have to make over £18400 in gain in order to pay tax.
when you say making £6000 a month. is that the gain amount? or what you receive from selling the shares?
if its the gain then you could be in for nasty tax bill, if its proceeds, then you need to calc gain by taking the cost of the shares away from the proceeds you receive.
obviously there is abit more to CGT if you own the shares for a number of years, such as taper relief and indexation if owned them before 1998, which would need an accountant to work them out for you.
another bit of advice, is to try and limit the shares you sell in any one tax year to either avoid paying cgt or to keep below the 40% tax band, so if you need extra cash, try to sell on 5th april, then on the 6th april as this is the finish and start of each tax year.
best bet would be to visit a local firm of accountants and they will take care of all of this for a fee. depending on how many transactions you have depends on how much it will costs.they should offer you a free no obligation consultation so see what you are doing and explain everything you need to know, then tell you how much it will cost to do you tax return form.