Does anyone fromUK especially have any thoughts about at what point its worth treating Mstock as a serious business for Tax purposes etc rather than just declaring the extra income?
The main and primary reason you should (besides having to tell HMRC of any extra earnings by law) is:
You can claim all expenditure on photographic expenditure including cameras lenses printers and lighting equipment against your tax burden.
If you have the receipts you can claim a capital allowance on equipment spend up to 5 years in the past.
I did and got a 1250 rebate from HMRC.
You can claim any other expenses such as business travel (0.45 per mile up to 10,000 miles 0.25 after that) printer paper memory cards accountant fees props and so on.
You can claim for books and magazines and if you advertise website fees advertising costs etc.
You can claim a reasonable amount for using part of your home for work i.e. spare room as an office garage as a studio space etc.
Of course you must run it as a business and keep copies of receipts invoices and bills. A simple concertina folder to store each month's invoices and receipts and a spreadsheet to keep track of incoming's and outgoings.
If you do work other than stock on a business to business basis it is worth registering for the "flat rate VAT scheme" as you can charge VAT on sales invoices but then retain a portion of the VAT to offset against purchases
http://www.hmrc.gov.uk/vat/start/schemes/flat-rate.htm#5So for example every 20 of VAT sales HMRC gets 11 of the VAT and you keep 9

And no you don't need to go over the VAT threshold of 75000 per year to register.
It's a lot simpler than the regular VAT process
http://www.hmrc.gov.uk/vat/start/schemes/flat-rate.htmI am registered on the flat rate scheme because I do business to business sales as well as stock and keep a good chunk of the VAT charged.
Alamy will add VAT to self billing invoices if you are registered and you can retain a portion of the VAT from those sales too.
It's also simple to operate you just record your sales and declare the VAT once a quarter and pay HMRC by direct debit.
I even saved myself 100+ per year by doing my own accounts but you might want to employ an accountant for your once per year tax returns.