Without seeing first-hand which is always useful as it may reveal stuff to a trained eye that a householder would miss, here's the general gist of why your signal falls down in bad weather. I know it's a bit of reading, but if you put in the effort (less than it's taken me to write it) you'll have a better understanding of what's going on in your system tso you don't waste time and money with ineffective solutions.
1) Your aerial is in the loft. That knocks a big chunk off the signal level is can generate, and that in turn means there's a lower signal;l to noise ratio which is a measure of signal quality. Quality is the most important thing, far far more important than power which can be generated with a simple booster. You can't do that with quality. Once it's gone it's gone
2) Digital signals are being shifted around to make way for selling off chunks of the TV bands for mobile communications. The result is that signals are getting pushed in to the lower bands. If you house had a Grouped Aerial that centred it's main sensitivity in the mid an/or upper bands, or a Wideband High-Gain aerial which again only work well in the upper bands, then they'll struggle when signals move to lower frequencies
3) You have an active splitter. This boosts strength but reduces quality. Don't worry, all powered splitters do the same. At least you have it up in the loft which is the best place as far as signal quality goes
4) There's a short cable run to the bedroom TV but a longer run to the lounge. All cables degrade the signal power. Good cables (all-copper ones such as Webro WF100, Triax TX100, Labgear PF100) have a minimal effect on the signal level and do nothing to harm the signal quality. Bad cables such as the cheap RG6 (steel core, aluminium shielding), old analogue 'Low Loss' coax or worse still those awful DIY indoor TV extension kits hammer the signal level - they're far more lossy - but also have poorer shielding. This means they're more prone to picking up interference, and that make the signal quality worse. So you have possibly the combined effect of more signal loss in the longer cable and extra interference because of poor shielding
5) Tuner sensitivity plays a part too. There's some variation, so one TV might go blocky when another working of the same signal will be fine
And finally...
6) When it rains the tiles hold water on their surface. Water reduces the signal level getting through in to your loft. Your aerial system then experiences a drop in signal level
Digital works on a bit of a knife edge for signal. We call it the Digital Cliff. A TV can be getting a signal that's close to marginal but the picture looks exactly the same as it would with a really good signal. Once the quality drops below a certain level that's when it all falls to bits. The weaker channels hit that point sooner than the stronger ones.
Those of us who install aerials for a living and do it right spend a lot of time making sure that signal quality is attained (good aerial, well positioned) and then preserved (good ancillaries, great cable) by making detailed measurements and planning a system carefully.
In your case then, unless there's some factors you haven't disclosed, then once you've done the basics of checking for loose connections and damaged cables then it's time to address the signal quality issue. If possible, get the aerial outside, or maybe even change it for one that's better suited to lower frequency signals.
