Lets assume a EV/EBITDA multiple of 8, so EV of $4bn. Knock off the debt and you get around $25 or 1600p per share. Obviously there are lots of execution issues to get through, but I think this is a reasonable target if the company can execute.
In the interim statement, the company repeated that, in the longer term (presumably with Hylas 4 full), EBITDA would be $500m. Current debt is $520m, and there will be additional debt for Hylas 4 of $125m - so $645m in total. After the placing there are 136.3m shares in issue.