wow! you got bitten! - I too started with a £15 mahalo, and quickly upgraded, (though if I am honest, I have my Mahalo in one of our bathrooms and it's used as a (ahem) distraction when sitting at the great white telephone.....)
In answer to one of your questions, if you spend more do you get better sound? - YES! - solid top, or even better, all solid Ukes have much MUCH better sustain and tone than your Mahalo will give you. Some of the very high end ones will blow you away. Same as acoustic guitars really. Better strings will also impove things no end. My original Mahalo came with what I can only describe as being one step up from rubber bands - get some Aquilas or Worths on it.
That said, your price point is similar to mine for my second Uke, and seems a sensible step up ( you will regret it though, you are on a slippery slope!!)
For me, my second Uke was a Flea from the Fluke Company (soprano) - it is sturdy (hell, you could play cricket with it), and has tons of volume and a nice bright tone - this served me well for a year, and I still love it. You may find one for a shade over 100 in its stock format (I think mine was 120) A Great great Uke - brillantly made in the US, keeps in tune and is a real talking point too.
If you cant shift on price - look at Kala and Ohana - but try to go for solid top ( you should get one at that price) - Southern Ukulele Store on ebay are great on price and have a whole range.
A £100 uke will be a huge step up from what you have, but you may still get niggles on intonation, buzzes, action etc (maybe). You wont get this with the Flea at all (as the fretboard is molded plastic - perfect dimensions - sounds odd, but works a treat), but on some at £100 made of wood, they are likely to be chinese and can suffer from quality control ( I stress "can" - some are wonderful instruments))
Next step up from there is into all solid territory, for which you are looking at £150 plus. I just had my all solid Mainland delivered which is a real beauty, but is £170. You could go to several hundred on this route.
(For my two penneth, I'd ignore comment regarding electro Uke - a nice plus I suppose, but if you factor it into your budget, you will get less Uke for the money. I have played acoustic guitar for 20 years, and have electros and non-electros - I have gigged with my non-electro, and have recorded it - if you want to do it, you do it - you use a Mic!!)
Finally, as for scale length - I started on a Sop Mahalo, my Flea is also a Sop. In moving on again to my Mainland, I went for a Concert - slightly bigger body, but more importantly, more frets- essential if you want to pick right up the neck. I have heard that concert scale is now becoming the most popular choice - to be honest though, I am not feeling "uplifted" by having a new one in Concert mode compared to my last 12 months with a soprano.... I dont agree that my concert sounds less "punchy" - seems to me to be the same as my Sop but with a longer neck!
Baritones and tenors are bigger again, and a different kettle of fish I cant comment on. In my view, Sops are fine, but I suppose Concerts are a little more flexible.
My summary - For £100 - I'd think about stretching to standard Soprano Flea, or look for a solid top Ohana or Kala.
All of that said - would totally support the earlier post regarding a Mainline - I have just had one delivered - was £160 - chinese parts, all solid top back and sides, assembled with love and care in Nashville by Mike who is on this forum. Stunning to look at, lovely warm sound - about as good as it gets as I can see at this price point (considered by many to blow away Ukes costing twice as much - if you can search for the extra £60, you should be delighted)
Hope that helps...
P