The best type of property to buy would be either a 2 / 3 bedroom home or a 2 / 3 bedroom apartment. The most crucial aspect to buying a property to rent is the location. It is important that the property has amenities close by such as schools & shops.
Back gardens are advantageous to tenants with small children, alternatively a large patio/balcony will suffice.
It is important to be aware of the BER cert and the implications for tenants with heating bills, try and buy something with a good BER or a property that you can upgrade the insulation / heating system in without great expense.