The scientific name for vitamin D3 is cholecalciferol. You do not need a prescription to buy vitamin D3. Vitamin D3 is also available in supermarkets, pharmacies, health food stores, and online websites. The upper tolerable limit of vitamin D is 4000IU daily. There are vitamin D3 supplements that contains 1000IU per capsule.
If you do take higher doses of vitamin D you must take into account magnesium. Vitamin D "uses up" magnesium to convert to active vitamin D in the blood. Magnesium is the most important co factor for vitamin D absorption. Vitamin D is also essential for calcium absorption.
You can convert µg (also means mcg or microgram) to IU (international units). IU is the most commonly seen unit of measurement for vitamin D supplementation however.
International Units and micrograms
40 I.U. = 1 mcg
400 I.U. = 10 mcg
1000 I.U. = 25 mcg
50,000 I.U = 1250 mcg or 1.25 mg (milligram)
The upper tolerable limit of vitamin D may be 4000IU, but this information is from the US vitamin D council:
"Vitamin D Council recommended amounts
Based on the body's indicated daily vitamin D usage, Vitamin D Council recommends the following amounts of supplemental vitamin D3 per day in the absence of proper sun exposure. Due to the variable response discussed above, these are only estimated amounts.
Healthy children under the age of 1 years – 1,000 IU.
Healthy children over the age of 1 years – 1,000 IU per every 25 lbs of body weight.
Healthy adults and adolescents – at least 5,000 IU.
Pregnant and lactating mothers - at least 6,000 IU.
Additionally, children and adults with chronic health conditions such as autism, MS, cancer, heart disease, or obesity may need as much as double these amounts."
As you live in London i had a quick google search to see if there were higher IU doses per capsule and found this from The vitamin D company:
"1. Vitamin D3 5000 IU--200 Micro Tablets (6+months supply)
This 5000IU formulation is suggested for the elderly, obese, pregnant women, those with dark skins and those with medical conditions where maintaining an optimal level is important. These tiny tablets are easy to swallow or can be crunched up in the mouth as they disintegrate easily and are virtually tasteless. Suitable for vegetarians.
Cost: £18.50+5.00P&P=£23.50 Discounted price: £15.50 FREE P&P No additional charges during check out!