Secondly, you need to assume that there are obstacles up to at least 800' above ground level. This is because obstacles below 300' AGL are not shown, and elevations change in 500ft. Therefore an undisclosed obstacle can be up to 798' AGL and not be shown on the chart.
Simply not true. Obstacles more than 300' AGL will be shown (unless the CAA omitted them in error).
Elevations are shown as spot heights, and don't change in 500' increments as you state.
Relief elevations are shown in bands (SL-500', 1000'-2000', 2000'-3000', 3000'-4000', 4000'-5000'). Obviously not all bands are needed on each chart
The highest elevation shown will be annotated in the key at the foot of the chart, along with its location.
So if the elevation at a geographical point is actually 1001', it will appear in the 1000'-2000' band colour. As a raw calculation, a pilot could then use that to assume the maximum elevation is 1999', then add on the maximum uncharted obstacle height of 300'. Or assume an obstacle elevation of 2300' in realistic terms. Of course in reality, the maximum elevation (including unknown obstacles) is only 1301'.
To provide a bit more accuracy, the CAA also provide Maximum Elevation Figures for each part of the mapping grid. This takes out the excessive 'rounding up' witnessed in the simple calculation above. The MEF uses the highest known feature in the quadrangle, including terrain and known obstacles, plus allowing for unknown obstacles (up to 300'). This is then displayed as a two digit code, denoting thousands of feet and whole hundreds of feet.
As an example, the MEF for the quadrangle containing Dublin is
31. This is based on the highest obstacle, a mast with an elevation of 2890' (380' AGL), plus 300' for unknown obstacles. I assume the CAA round down to the nearest hundred when making this calculation. I guess their logic is that the extra 90' (in this example) should not make a great difference to a VFR flight !!