First thing first, this purely depends on where you guys are based. If you're based in Silicon Valley you could potentially raise more money at a better valuation. My experience however is raising money in Europe, specifically in the UK and Portugal, and although I have raised from angels across the world from places like China, Singapore, US, and the Middle East, I will speak to the mentality of European investors.
At the stage you are at right now, I would suggest going for a 100,000 GBP round. Whatever amount you decide on, DO NOT CHANGE IT. Throughout the entire fundraising of this round (whether it is one week or two months) make sure not to change the asking amount. So be very picky with this number.
Second, set a minimum investment amount from each investor. Since you are dealing with a seed stage round, I would suggest to go with around 10k GBP (so max you end up with 10 investors with minimum amounts). If you feel that is too high then drop it to 5k, but then also bring your asking amount down to 50k.
Valuation range for your stage and startup (in Europe only) would be less than 1 million GBP (around 500k to 800k). If you're lucky and have a ton of interest from investors and can create competition among them, you could potentially get up to 1.5 million GBP. This means that if you're raising 100k GBP as a seed round, you'd be giving up between 10% to 25% depending on your valuation.
I would recommend not giving up more than 25% in a seed round, and know from experience that unless you're lucky or dealing with investors who don't know what they're doing, you will need to give up more than 10%.
I'm happy to speak in more detail about this if you feel it would be helpful.