Using public data from OFCOM and the CloudRF API, here is a consolidated coverage map for Vodafone's UK mobile GSM 2G 900MHz network built from 22,000 distinct sites in a spreadsheet.
The map below was created using 22,069 of Vodafone's 900MHz cells and due to a lack of information some parameters such as antenna gain were assumed with omnidirectional patterns. The receiver sensitivity chosen was -80dBm which equates to an average signal on your phone. A mobile can receive a signal at down to -110dBm but this is really not reliable for planning purposes where interference and fade losses should be factored in. Radius per site was capped at 25km and an average random clutter height of 4m was applied to compensate for the lack of ground clutter (buildings) in the SRTM terrain data.
Finding an independent national coverage map for a mobile operator is practically impossible given the lack of current data. The best option is an app like OpenSignal which uses empirical crowd sourced data to show coverage - for the locations of other app users.
Operator created coverage maps are unsurprisingly optimistic as they are a powerful sales tool but these 'good news' maps always lack key details such as receiver sensitivity eg. Is the map for a 5 bar signal or 1 bar? and fade margin eg. will coverage be that good all year round with different climates? and most importantly the uplink from the handset to cell tower - all those operator coverage maps are for the downlink only.
The data isn't great or current (~2012) as the OFCOM disclaimer makes clear but is the best available publicly. OFCOM has encouraged operators to voluntarily share network data since 2005 but operators ceased sharing following a ruling in 2007 that this was public data which must be published. The operators claimed their data would give a competitor a commercial advantage - a plausible excuse.
Of note many data records were unusable due to incorrect grid references and analysis of the supplied power output for some operators suggests human error again.
The 2G coverage depicted is based upon old data which contained errors and does not represent the total 2G/3G/4G coverage of the current Vodafone network.