Architects practices, depending on their scale, often advertise in local magazines, attend network meetings, and perhaps advertise in the yellow pages and construction periodicals for their business marketing.
This way of doing things has paid off in the past and can also be enhanced by having a website to refer people on to once they have seen the ad, flyer or business card.
A few photos showing some portfolio work, such as different aspects of building construction, project plans, 3D visualisations and artist impressions amongst finished detailed jobs would give the viewer some trust in previous work. It would be a good idea to have certification images from architect practicing accredited bodies and anything else that shows the architect practice to be desirable for contracting.
A contact form could send an enquiry to a specific email address. This will ensure a system of archiving potential leads.
Keeping track of old contacts and reminding them of your presence with news of recent projects & future work are some suggestions that would work really well with a promotional strategy.
An architect practice could be fairly broad reaching and could work on projects based all over the UK. Depending on the type of business the Architect requires would be a good basis for an Search Engine strategy mixed with some local terms if targeting a specific regional area is required.