A good installer will provide you with a written quotation for the work beforehand, and not require payment until the work is complete. Good companies provide identity cards to staff that work in customer's homes.
It's often best to ask friends and neighbours for recommendations too. They may know, for example, if you need a high-cost class I aerial or if there is a more economic solution.
The aerial should be installed in a place that is good for reception and complements the building. Ideally, it should not be visible from the street. If this is impossible, it should not distract from the building. If you live in a listed building or conservation area you may have other restrictions.
Any existing aerials should be disconnected. If the building has multiple aerials any new aerial should be around one metre away from any existing aerials. If the signal is poor, it may be more economic to have a single high-quality aerial and a mast-head amplifier, rather than several poorer aerials.
Mounting many aerials on the same pole will degrade the signal received by each, unless they have about one metre spacing.
The cable that connects to the aerial can be a variety of colours; the one chosen should be complementary. It must be secured at regular intervals and travel either vertically or horizontally. When it enters the house from the outside, this should be done directly though a hole drilled right though the brickwork, and then sealed. A hole though a wooden window-frame is OK but one though double-glazing is not.
The cable inside should be tidy and run horizontally or vertically too, and be securely and regularly fixed. The cable should end in a wall-mounted socket within a few metres of the television set.
From this socket another fly-lead connects to the TV or set-top box. In the event of a thunderstorm, the equipment can be protected from random lightning damage by the disconnection of this "fly-lead" cable.
You should expect a installation for Freeview reception to give the full range of channels with uninterrupted video and sound on all channels once connected.