Taking a look at the radar imager for 1100 Local (1000 UTC) on the Met Office public web site (sorry, haven't figured out how to insert images), shows rain/showers just north and west of Southampton - and not very far from Bournmouth at all. FFF I take "won't" as meaning 0% possible, but really with rain so close (and also some further NW) I don't really believe the possibitlity was 0%. Seems to me they knew showers would develop in the area, but cannot predict the track of every one.
Southampton reported -DZ and -SHRA at 1020 UTC And 1050 UTC.
Seems to me Met O were alerting to a realistic alternative, without going overboard and forecasting truly awful vis and drastically lowered cloud base. In fact the cloud cover was very variable at Bournmouth, and actually lower than the 0716 TAF predicted, but all within the 'leeway' allowed for in ICAO's and CAA's rules.
EGHH 071350Z 28013KT 9999 FEW010 BKN021 16/12 Q1008=
EGHH 071320Z 29011KT 9999 FEW010 BKN021 16/12 Q1008=
EGHH 071250Z 32013KT 9999 FEW018 BKN022 16/12 Q1008=
EGHH 071220Z 31011KT 9999 BKN021 16/12 Q1007=
EGHH 071150Z 31010KT 9999 SCT022 BKN028 16/12 Q1007=
EGHH 071120Z 31012KT 9999 SCT021 BKN022 16/12 Q1007=
EGHH 071050Z 31012KT 9999 SCT021 BKN028 16/12 Q1007=
EGHH 071020Z 31014KT 9999 FEW009 BKN022 16/12 Q1006=
EGHH 070950Z 30011KT 9999 FEW009 BKN019 15/12 Q1006=
EGHH 070920Z 30013KT 9999 FEW010 BKN019 15/11 Q1006=
EGHH 070850Z 29014KT 9999 SCT024 15/12 Q1005=
EGHH 070820Z 30013KT 9999 SCT030 15/12 Q1005=
TAF EGHI 071159Z 071322 26008KT 9999 BKN025=
TAF EGHI 070906Z 071019 25008KT 9999 BKN025 PROB30 TEMPO 1016 8000 -SHRA=
TAF EGHI 070638Z 070716 29013KT 9999 SCT020 TEMPO 0714 28015G25KT PROB30 TEMPO 0716 8000 SHRA=
EGHI 071150Z 26006KT 230V300 9999 FEW020 BKN024 14/11 Q1006=
EGHI 071120Z 26006KT 230V300 9999 FEW020 BKN024 14/11 Q1006=
EGHI 071050Z 26006KT 230V300 9999 -SHRA FEW020 BKN024 14/11 Q1006=
EGHI 071020Z 25006KT 220V290 9999 -DZ FEW018 BKN022 14/10 Q1006=
EGHI 070950Z 25006KT 220V290 9999 FEW018 BKN022 14/10 Q1005=