Obviously lower frequencies go further distances but realistically are you actually going to notice the difference in signal strength on 87.5 than you would on 108 ?

Not really, unless your tx amp isnt linear or is tuned for certain frequencies, and of course sone of it is down to the antenna for transmitting, and the antenna for receiving

The difference is 1.8dB. The receiver hardly notices the difference.

on the 7 watt one I find lower part is better

Guess not much in it.

Potential of interference from strong legals on 107.9/8 in some part of the UK.

87.5/6 is not used by legals, so a clear frequency for distance / dx receive. Apart from rsl's on 87.7, if they are close.

im just using 108 as an example as its the furthest frequency away. i mean anything further up compared to the lowest you can possibly go.
also i would never touch 87.anything because you never know when some tiny rsl will switch on it and they start moaning your going over them.

Noise/interference (from switchmode power supplies etc) on average tends to be a bit worse at the bottom of the band than the top, so its a 'much of a muchness'
it probably cancels out the slight benefit in propagation in a low FM band frequency over hills etc

The "noise" lower down the band is more due to the huge number of high powered regional transmitters below 95 MHz, rather than any effects from SMPSUs and the like. Most power supply noise is (usually) around 200 - 500 kHz, with some of it (usually harmonic rubbish) turning up all over the shortwave bands.

It's instructive to connect a spectrum analyser to a reasonably broadband aerial, and examine Band II. The lower part of the band has a noise floor that's 6 - 10 dB worse than the upper part of the band. You can see all the huge number of regionals all on top of one another - though not as they are in some parts of the world where they use just two adjacent frequencies for national stations.

Back in the late 50s, when VHF FM broadcasting was just starting in the UK, the only stations were three BBC outlets - "Light", "Home" and "Third" ( later Radios 2, 4, and 3 respectively). Domestic receivers in those days used VFO tuning (no PLLs back them) and "AFC" on the better ones to keep them tuned. Sensitivity and selectivity of those early domestic receivers were both relatively poor, so the BBC ran enormous power, and spread each station on multiple frequencies across the lower part of the band.

When local, then Commercial radio began (in the late 60s and early 70s), frequency allocations were such that the BBC still spread across half the available band with their national outlets, and the "little" stations (each running several kilowatts!) were spread far apart on the upper part of the band - though there was nothing allowed above 98MHz, because the Police used ~99MHz upwards! It was still assumed that FM receivers would have poor selectivity.....

Even with the advent of modern PLL receivers and good IF filtering, the BBC still insist that they "MUST" occupy >50% of the FM band. It's now just purely political - ask OFCOM for a licence and they'll STILL tell you that there are "no frequencies available"! There hasn't been any reason for BBC Radio 2 to spread from 88 to 92 MHz for at least the last 40 years. Proper engineering could move all the national stations to a sub-band below 92 MHz - they could use "salt and peppering", just using a pair of adjacent frequencies only 200 kHz or 300kHz apart for each station throughout the country - "capture effect" takes care of any interference issues.

Back in the late 70s, I was involved in the "Wise Report", where a prominent broadcast engineer (Fred Wise) carried out an exhaustive feasibility study to band-plan Band II. We suggested "Nationals" below 92MHz, "Regionals" from 92 - 95MHz, "City-wide" from 95 - 102MHz and "Community / Special Interest" for the rest of the band. A big city like London could - easily - support over 65 stereo stations with 300 kHz channel spacing, and with intelligent location planning, physically adjacent stations could be 600 or even 900 kHz apart, eliminating interference issues..... We would never have needed the disastrous DAB!

DAB is made i the same reason as you mention... political and businesses.. produce more garbage pollution Nta also...