Although in the UK we all benefit from free healthcare on the NHS, taking out a health insurance or PMI (Private Medical Insurance) policy can provide you with extra support, convenience and comfort should you need it.

Private medical insurance shouldn't be seen as a replacement for NHS treatment, but rather an add-on to complement the care you automatically receive.

Private health insurance generally covers the cost of in-patient treatment - things such as tests, surgery and day-care - and out-patient treatment. However, every policy will vary considerably in what is covered and what is excluded.

It's vital that you compare health insurance UK wide to make sure you get the service you want, and not just the price. How comprehensive a policy is will be the major determinant of price, but in general there is a basic level of cover offered by even the cheapest policies.

What's covered by health insurance?

Few if any private medical insurance policies will cover treatment for illness or injury pre-dating your cover. Don't buy any policy retrospectively unless it specifically declares cover for pre-existing conditions.

Medical insurance is generally intended for short-term, curable illness or injury (known as 'acute'), rather than ongoing conditions, and more often than not for things that can be cured without lengthy medical procedures.

By taking out private medical insurance it's possible to cut standard waiting periods, choose where and how you are treated, and sometimes get private rooms as opposed to a bed in a ward.

The idea is that through these measures your treatment for and recovery from these conditions will both be as quick as possible. Your doctors are likely to be NHS rather than 'private', and you'll often get the same treatment as you would on the NHS - just more quickly.

However PMI doesn't only cover minor conditions; many policies will cover things like diagnostic treatment, radiotherapy and chemotherapy, physiotherapy, as well as provide private ambulances, discounts towards annual check-ups, and home-nursing.

It is important to compare health insurance policies thoroughly to identify exactly what is offered, and equally important to distinguish between customer perks and things that will directly impact on your health provision.

For example, many health insurance policies will provide in-patient cover, while some will provide out-patient only. Others still might cover a combination of the 2. Some policies will also give you cashback when you spend a night in an NHS hospital, which goes some way to mitigating the costs associated with unexpected illness and injury.

Some policies also cover what's known as 'alternative treatment' which could include homoeopathy, the services of an herbalist, or acupuncture. The NHS tends not to provide these services so private medical insurance can be a great way to lower the cost of such treatments.

What's not covered by health insurance?

Medical insurance is usually intended for short-term illness or injury rather than for conditions that are ongoing ('chronic') and/or incurable: it won't generally cover any pre-existing or ongoing conditions that you already have and are receiving treatment for, such as diabetes and asthma.

Because of this it tends not to be possible to take out health cover for treatment you know you'll need in the near future, or that isn't medically necessary. This will vary by provider, so be sure to compare PMI policies and companies.

Most providers will exclude treatments that you choose to have yourself (known as elective treatment) such as cosmetic surgery, and also exclude fertility treatment, treatment for pregnancy, HIV, or psychiatric conditions, as well as some experimental treatments.

Whichever health insurance policy you do choose, it's vital to read all the conditions and exclusions so you know exactly what you're getting and what's covered.

The best way to go about choosing the right policy for you is to compare health insurance providers and get as many quotes as possible.

Read our guide on buying health insurance to find out all you need to know about taking out a PMI policy.