The official source for information on death records is the Office of National Statistics (ONS) and there are stats available with monthly figures and weekly figures.
When you look at the weekly total deaths figures for 2019 for example, you notice a drop in numbers during the last week of May, the last 2 weeks of August, and Christmas week.
When you look at the monthly figures, the figures are pretty consistent except there is a much higher figure in January and there are lower figures in June, August and September. Strangely, July's figures are fairly consistent with the rest of the year.
based on the actual number of death registrations recorded for each corresponding week over the previous five years. Moveable public holidays, when register offices are closed, affect the number of registrations made in the published weeks and in the corresponding weeks in previous years.
This could also explain the high figure for January as Christmas will shift some of the December figures into January.
The weekly stats spreadsheets downloadable from the ONS give figures for
All respiratory diseases (ICD-10 J00-J99) ICD-10 v 2013 (IRIS)
The trouble is that without access to ONS data held within their IRIS software, deaths by cause is difficult to ascertain for causes other than respiratory diseases, and you cannot make any meaningful data comparisons with the ONS sheets on suicides as they are recorded in annual figures rather than weekly or monthly.
As far as I am able to find, there is no meaningful information that can be sought regarding causes for the dips in the figures you mention