Can the address be used to determine if a property is a flat?

UK addressing is complex and there are no hard and fast rules.  Postcodes can include a mix of addresses as you say including residential and non-residential addresses.  Commercial addresses will generally have an organisation name (but not always) and may be a standalone address such as a shop or can be an industrial estate or shopping centre which is subdivided into units with the unit information generally in the sub_building field.  For residential larger properties (detached and semi-detached) we will typically have a building number and/or name, some properties may be converted to flats and sub-divided and may have flat numbers (but this will not always be the case e.g. if the property has only one shared postbox for all properties inside).  Flats will generally have a building number and/or name of the block e.g. (Flat) 6 Richfield Court, 3 Hayne Road, Beckenham BR3 4HY. The word Flat is in brackets as it is common to use the address with or without mentioning flat.  It is split like this:

Sub Building: 6
Building Name: Richfield Court
Building Number: 3
Street Name: Hayne Road
Locality: Beckenham
Postcode: BR3 4HY

This is typical for private purpose-built residential blocks of flats.  For some other cases, often social housing but not always all of the properties on the street are flats and the local authority have addresses these with building numbers rather than sub buildings as they are not associated with a particular block or building name but rather every property on the street is a flat and has a sequential building number.  This is quite common with maisonettes which are a special type of flat with 2 stories, often arrange in blocks on top of one another.

To conclude you cannot rely on the address to determine what type a property is.

This is why we have built our Addresscloud Property dataset which Lemonade have access to as part of the contract.  We take data from lots of sources including surveys, property listings, valuations and government sources modelled out using geography to determine property characteristics: https://docs.addresscloud.com/#addresscloud-property-acloud