I have just found the following in the Clinical Knowledge Summaries on the NHS Direct website
here.
Infection
There is a risk that you may develop an infection after your surgery. This can usually be treated with antibiotics.
In rare cases, a severe infection can require removal and replacement of the artificial joint (revision surgery).
You may also need to take preventative antibiotics before dental work or other surgery in the two years following your knee replacement. This is to prevent an infection developing in your replacement knee. Speak to your doctor or specialist for advice.
This is similar to the advice given on About.com: Orthopedics
here, which I think is a US site. This also makes special reference to patients, like myself, with RA.
Immunocompromised/Immunosuppressed Patients
These include patients with rheumatoid arthritis, lupus, and other conditions that can affect the immune system. Patients who take medications or receive radiation treatment that can alter the immune system should also have antibiotics prior to dental work.
Now, in the UK who decides whether or not to prescribe you anti-biotics - your dentist or doctor? Perhaps Jo will know.