I agree that Google Adwords may potentially be one of your most effective marketing options. Use your keywords in the headline and text of your ads and ensure that you use partial and exact matching for your keyword phrases as these will increase your clickthrough rate (CTR) and lower your costs. [exact keyword] "partial match keyword".
You can use negative keywords to exclude any words and qualify your prospects (put a minus sign before the word you want to exclude). Try and be as specific as possible in your keyword phrases, focusing on phrases of 3 or more words, rather than very general 1 and 2 word phrases. These will have fewer searches, but should generate far superior returns/conversion. Adwords also has the option of targetting specific countries, so make sure you target UK only or UK and France only - every day I see ads showing on google.co.uk from US advertisers who don't deliver outside the US! You may also wish to set a daily budget to control costs. If you don't yet have an Adwords account, PM me and I will send you a link for a free £20 voucher so you can open one and have a test run at no cost.
As for the forums idea mentioned above, I know that French Entree is popular and an excellent site which also has a discusssion forum -
http://www.frenchentree.com/ - but as was pointed out be very, very careful not to advertise blatantly (spamming) - post useful advice and perhaps only mention your site in your signature or profile.
French Entree also offer various advertising and sponsorship opportunities which you may want to consider:
http://www.frenchentree.com/fe-advertising/DisplayArticle.asp?ID=1593
There is also a magazine called a Place in the Sun (they have a general 'buying property overseas' magazine and a France specific magazine) - display advertising may be costly, but there are probably good opportunities via PR for getting mentioned/featured in the magazine at no cost. They also list and advertise overseas property exhibitions held around the UK - these may represent a good marketing opportunity for you?
Best of luck - it sounds like an interesting site and the market is certainly there, perhaps you will post a link when it is ready? Also, if you can elaborate on what product/s or service/s you offer, I can provide more specific online marketing advice.