You can get documents notarised at most Solicitors (their company name will usually say "Solicitor and Notary public" or similar).  There's a fixed fee for it (varies per So;icitor) - usually in the £25 - £75 range.  There are other people besides SOlicitors who can notarise - but SOlicitors are liekly to be the most accesible ones for you.  Check for any extras they charge before getting it done - you don't want to pay them £1 per sheet for photocopies for example.  You'll need to take along proof of identity (passport is ideal) and the originals of any ID documents being copied/sent (they MAY insist on photocopying the documents themselves).  You'll then swear under oath that the information contained in the affidavit is correct.

You then need to get the documents apostilled (which means a central body confirms that the notarisation is valid).  This has to be done by post with the Legalisation Office of the Foreign and Commonwealth Office (you used to be able to get it done in person at Milton Keynes but that option is longer available).  Think the cost of apostille is £35 per document from memory.  Just google for "apostille UK" and I'm sure you'll find the details for that.

It's perfectly legal to copy documents such as passports.  They are explicitly listed as things which can be apostilled - and apostille is pretty much only used when sending documents overseas.  I have no idea if other countries have laws against sending copies of passports - but it's fine from the UK.

My objection to this procedure has always been that the costs are disproportionate for smaller investors and that they're being passed on to investors when the mistake was Giga's.  There's nothing illegal (in the UK) about what is being requested - and were disclosure of the requirement for such documentation made in advance of accepting funds it would be perfectly reasonable.