Very hard to do anything I'm afraid.
1. You have no control over what is sent to you (what your router receives) so if someone can send a load of garbage at your connection and it fills your available bandwidth completely, that's what will (and does) happen.
2. DDoS tends to be from several (hacked/compromised) hosts which are part of a DDoS network. This could be a single host (IP address), or it could be hundreds of hosts (hundreds of IPs) spanning all parts of the internet, working in unison to create the attack.
3. Even if you could, sending garbage traffic to these hosts achieves nothing as it is highly unlikely that will not reduce (in any meaningful way) the garbage traffic they are sending to you. Also, the bandwidth you have available to you outbound is very minimal in the grand scheme of things, for any UK broadband connection currently.
Only option is to go with a provider who can offer dynamic IP addressing for your broadband connection, so you are issued a new IP address each time you connect. I'm also curious how who ever is DDoSing you is finding out your IP address. If you have a static one, then they may have just acquired it by some means (you told them it, they located you in a server log file somewhere, and because it's static, they know this IP will always be you and thus have an easy target). If you have a dynamic IP, it could make things a bit more trickier for them. If they are finding out your IP address from the gaming server you are connecting to (eg. the server discloses this information to all players), then going the dynamic IP address route is also going to be a complete waste of time.
I have no further positive advice to offer you. DDoS is a problem, and will continue to be so for years to come. When these attacks were prevalent a decade or so ago during my days on IRC, a trick I found was to firewall my connection in such a way that I limited the amount of reply traffic I sent back out during such an attack. This worked well to a point, but in the end, if the attack lasts long enough, your connection will eventually suffer and become unusable as a result.