Anyone can be Self Employed and Employed at the same time, it all depends on the facts of the engagement.
Basically if you act as a "servant" to a "master" then you are employed, if you provide services to anyone who will pay for them you are self employed (Assuming you are not a Company Director).
The exemption (Capital Gains Tax) is currently £10,000 per year, but it relates to gifts of cash , assets, investments etc, not items provided to dependants.
If you are Self Employed business expenditure is deductible, but debts are not, although interest on loans that qualify for business expenditure can be claimed. The HMRC Website has lots of FREE advice and leaflets to download