If you have a SIPP, you can invest in broad range of investments: private company,basic investment trusts, OEICS & unit trusts to Commercial property including development land, traded endowments and Stocks & shares, both quoted and unquoted companies. The answer is yes .SIPP funds to make an investment in up to 20% of a Private Limited Companies shares..but it also depends the SIPP trustees if they are comfortable with the decision.

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Sorry if my initial comment sounds too little but i guess it sorts out your initial question ie whethe you can invest in a private company with your pension.

"Am going to move my pension to a sipp then invest it in a friends business or manage it myself in a forex account whcih am not sure of but i will kind a company to set it up as a sipp" - could you please clarify your further comment.

A sipp is just a tax wrapper. You choose a company to administer your sipp and they transfer your pension into it. If you need to buy certain investments, forex, private company etc -there are some general guidelines on what % to invest but the sipp provide will have a say. Basically, the idea with a sipp is that IT IS NOT YOUR MONEY. It is monies held in trust (by your sipp company) with you as the beneficiary but you can instruct where it is invested but in agreement with the other trustees .

Many SIPP providers would prefer you invest within a controlled investment system, maybe using their inhouse ,say forex trading platform, or share trading platform ..because they are trustees and would want to ensure some control as they are to some extent liable for your investment decisions.

What you need, if you want to trade forex or invest in a private company, is a sipp provider who will allow you to invest in these things - provided it meets inland revenue rules. Once you open a sipp with a company like this you will be able to decide what YOU want to invest in and then instruct the company to open a forex account with OANDA, FXCM, City Index (if that's who you've chosen) in the name of your sipp or transfer the funds to your private company. However there are few such SIPP providers who will offer forex trading due to some UK SIPP laws you cant trade at margin with SIPP so you need 100% of the trade or allow you to invest in some private company in which you are a connected person.

As said before on your other request/question - now closed, some countries however have less strict rules on who can be a trustee on your SIPP. Offshores like Channel Islands in particular allow you to appoint family members/ friends/ etc (if they are local) to be your trustees and you wont need a SIPP provider.That way you can invest in any assets/ borrow from pension as long as your Trust deed allows it (you draft the trustee deed). However, again as said before, you risk misusing the pension, if you have too much control. If you need help setting such an offshore SIPP i can help as im based in channel islands.

Here are few SIPP provider names you can try:
https://www.epml.co.uk/sipp
http://www.jameshay.co.uk/SIPP/Explained.aspx

The same principle applies to investment in private company. You need to a sipp provider who will allow you to invest such an investment.

Hope this helps.