If the only thing that tempted you to choose Exeter instead of Royal Holloway was because overall it's more reputable, then I think you've made the right decision to go to RHUL instead, since you prefer their location, campus, course structure and course reputation.
There may be some truth in the whole overall reputation debate, especially if you're going to find a job in future that's not specifically related to your degree. However, if you want a career that's related to English/Creative Writing (I dunno, journalism?) then your prospective employers would know which unis are best for that particular course. In other words, a degree at RHUL would be much better than Exeter. But tbh I think this issue of reputable uni vs. reputable course only really affects your first job, after that employers tend to look a lot more at what you've done in your first employment rather than where you went to uni, and eventually where you went to uni wouldn't even matter.
Since you've firmed RHUL anyway, there's no point worrying about it now (even before you've started!) It's certainly not disastrous and it seems like you'd enjoy it a lot there =)
Source: Myself, I've had lots of trouble deciding between Nottingham, Warwick or Gap Year + reapply!