I have just looked again at the specs on the manufacturers website for your aerial and tried to find the destructions for uour signal meter but unfortunately can’t find them sorry. So I’ll put my electronics and electrical head on and try to gauge what I think you are getting.
Firstly Screwfix inflating the gain figures for the aerial by quoting 14dB, what they haven’t mentioned is the measurement is 14dBi (decibels relative to isotropic radiator), this simply means is the aerial output is the same as the input which is impossible and therefore impossible and wrong and simply a sales ploy. The manufacturer Quotes 13.5dBi.
dB is a ratio of an amplifier or splitter etc, simply the ratio between the input level and output level. Where and aerial is concerned the gain should be measured in dBd (decibels relative to the dipole, simply the gain ratio to the dipole on its own).
To convert dBi to dBd you simply deduct from memory 2.15 (its 2. something) which equals 11.35 dBd peak gain of the triboom aerial.
None of this matters of course if your aerial works, I just thought it was worth a mention.
Moving onto your signal meter readings: µ
dBu = electrical field intestity – which is measured above 1 microvolt (mv)/meter (dBv/m)
dBuv = voltage in dB above 1 microvolt, into a specific impedance – in our case 75ohm.
dBmv = power level in dB above 1 milliwatt.
The “u” in dBuv (/m) refers to a back to front y as in the greek letter µ
So on the understanding your signal meter is measured in dBuv – which is relative to 1 microvlt. Which can be used to calculate signal levels throught the whole system, including at each TV input.
Where 0dB = 1uv (1 microvolt) and is worked on an logarithmic scale using the the calculation 10dBuv/20. for example 20dBuv= 10uv (microvolts) but 40dBuv= 100uv (mirovolts)
So in your case according to your meter: 5dBuv = 1.8uv (microvolts)!!
For digital TV preferable to have 45-60dBuv at the TV input and more at the aerial (analogue was much higher). Idealy your after 50-55dBuv
So unless you your signal meter is a multiple of 10 either either on o or 1 setting you have a problem.
Few examples:
If you are in a strong signal area and you have 65dBuv at the aerial and loose 5dBuv because of the coax run you have 60dBuv at the TV input thats fine, although I’d fit a 6dB attenuator to bring it down to 54dBvu at the TV, as too strong a signal risks burning out the DVB-T tuner in the TV. (65-5-6=54)
Again if you had a 65dBuv at the aerial and lost 5dBuv through cable loss but, installed a 4way splitter loosing a further say 6dBuv, that would be fine because i’d still have about about 54dBuv at the TV. (65-5-6=54)
On the other hand if you were in a poorer signal area and had say 45dBuv at the aerial and lost 5dBuv through the coax and installed booster that bossted each port by 14dBuv again I’d have about 54dBuv at the TV input. (45-5+14=54)
On the other hand if your in a strong signal area and have 65dBuv at the aerial and lost 5dBuv through the coax and fitted a booster with 14dBuv at each port you would have about 74dBuv at the TV input and risk overloading the DVB-T tuner. (65-5+14=74)
(taking into consideration a little loss on the coax from splitter/booster to TV)